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Spectroscopic orbits and variations of RS Ophiuchi
Aims. The aims of our study are to improve the orbital elements of thegiant and to derive the spectroscopic orbit for the white dwarfcompanion of the symbiotic system RS Oph. Spectral variations related tothe 2006 outburst are also studied. Methods: We performed an analysisof about seventy optical and near infrared spectra of RS Oph that wereacquired between 1998 and June 2008. The spectroscopic orbits wereobtained by measuring the radial velocities of the cool componentabsorption lines and the broad Hα emission wings, which seem to beassociated with the hot component. A set of cF-type absorption lineswere also analyzed for a possible connection with the hot componentmotion. Results: A new period of 453.6 days and a mass ratio, q =M_g/Mh = 0.59 ± 0.05 were determined. Assuming amassive white dwarf as the hot component (Mh = 1.2-1.4Mȯ) the red giant mass is Mg = 0.68-0.80Mȯ and the orbit inclination, i = 49°-52°. ThecF-type lines are not associated with either binary component, and aremost likely formed in the material streaming towards the hot component.We also confirm the presence of the Li I doublet in RS Oph and itsradial velocities fit very well to the M-giant radial velocity curve.Regardless of the mechanism involved to produce lithium, its origin ismost likely from within the cool giant rather than material captured bythe giant at the time of the nova explosion. The quiescent spectrareveal a correlation of the H I and He I emission line fluxes with themonochromatic magnitudes at 4800 Å, indicating that the hotcomponent activity is responsible for those flux variations. We alsodiscuss the spectral characteristics around 54-55 and 240 days after the2006 outburst. In April 2006 most of the emission lines present a broadpedestal with a strong and narrow component at about -20 kms-1 and two other extended emission components at -200 and+150 km s-1. These components could originate in a bipolargas outflow supporting the model of a bipolar shock-heated shellexpanding through the cool component wind perpendicularly to the binaryorbital plane. Our observations also indicate that the cF absorptionsystem was disrupted during the outburst, and restored about 240 daysafter the outburst, which is consistent with the resumption ofaccretion.
| Kataklysmische bedeckungsveraenderliche fuer CCD beobachter - teil 2.
Not Available
| CI Cygni
CBET 1504 available at Central Bureau for Astronomical Telegrams.
| CI Cygni
CBET 1504 available at Central Bureau for Astronomical Telegrams.
| CI Cygni
CBET 1487 available at Central Bureau for Astronomical Telegrams.
| A pre-outburst of the classical symbiotic star BF Cyg in 1987 1989
We analyze previously unpublished spectrophotometric data for theclassical symbiotic star BF Cyg obtained in 1982 1989. These data showthat BF Cyg already started a new period of activity in 1987 1989,earlier believed to correspond to quiescence. An optical spectrum takenon September 29, 1987 displays several features characteristic ofactivity in classical symbiotic stars. During the same time interval,excess near-UV radiation was detected, possibly from the accretion disk,and evidence for Rayleigh scattering of the hot component’sradiation on atomic hydrogen was detected in the far-UV, at phasespreviously believed not to be characteristic of this effect. This latterfinding may testify to an enhanced (compared to 1979 1986) rate of massloss from the cool component of BF Cyg. We discuss the interrelationbetween an increased mass-loss rate of the cool component and the hotcomponent’s activity in the context of classical symbiotic stars.
| The remarkable properties of the symbiotic star AE Circinus
We present new optical spectroscopy and photometry, Two-Micron All-SkySurvey (2MASS) infrared observations and 24 yr of combined AmericanAssociation of Variable Star Observers (AAVSO) and Association Francaisedes Observateurs d'Etoiles Variables (AFOEV) photometry of the symbioticstar candidate AECir. The long-term light curve is characterized byoutbursts lasting several years and having a slow decline of ~2 ×10-4 magd-1. The whole range of variability of thestar in the V band is about 4 mag. The periodogram of the photometricdata reveals strong signals at ~342 and 171 d. The presence of theemission feature at λ6830 Å at minimum and the detection ofabsorption lines of a ~K5 type star confirm the symbiotic classificationand suggest that AECir is a new member of the small group of s-typeyellow symbiotic stars. We estimate a distance of 9.4 kpc. Our spectrumtaken at the high state shows a much flatter spectral energydistribution, the disappearance of the λ6830 Å emissionfeature and the weakness of the HeII 4686 emission relative to theBalmer emission lines. Our observations indicate the presence ofemission-line flickering in time-scales of minutes in 2001. The peculiarcharacter of AECir is revealed in the visibility of the secondary starat the high and low state, the light curve resembling a dwarf novasuperoutburst and the relatively short low states. The data are hard toreconciliate with standard models for symbiotic star outbursts.
| Aktivitaeten zwischen August und November 2008.
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| Rotational velocities of the giants in symbiotic stars - II. Are S-type symbiotics synchronized?
We have measured the projected rotational velocities (vsini) of the massdonors for 29 S-type symbiotic stars (SSs) using high-resolutionspectroscopic observations and the cross-correlation function (CCF)method. The results of the CCF have been controlled with syntheticspectra. The typical rotational velocity of the K and M giants in S-typesymbiotics appeared to be 4.5 < vsini < 11.7 kms-1. Ina subsample of 16 S-type SSs (with known orbital periods andwell-measured vsini), 15 have deviations from synchronization less thanthe 3σ level. This means that we did not find evidence for astatistically significant deviation from the synchronization for any ofthese 15 objects. The deviation from synchronization is statisticallysignificant (at confidence level >99 per cent) only for the recurrentnova RS Oph.For 22 S-type symbiotics we give clues as to what their orbital periodscould be.Based on observations obtained in ESO programmes 073.D-0724A and074.D-0114.E-mail: rkz@astro.bas.bg (RKZ); mfb@astro.livjm.ac.uk (MFB);andreja.gomboc@fmf.uni-lj.si (AG)
| 2MASS near-infrared photometry of symbiotic stars
We present Two Micron All Sky Survey photometry for 216 symbiotic andpossible symbiotic stars. This represents the most comprehensivenear-infrared photometry for these sources which has been published todate. The results are used to define the statistics of S- and D-typestars; to investigate the photometric properties of Magellanic S-typesymbiotics; and to define the possible evolution of D-type symbioticswithin the J - H/H - KS colour plane. It appears that thecolours of D-type symbiotics are consistent with differing mass-lossrates dM/dt, provided that the grains have a silicate-like composition,and maximum temperatures of the order of ~800 K.
| Infrared Spectroscopy of Symbiotic Stars. V. First Orbits for Three S-Type Systems: Henize 2-173, CL Scorpii, and AS 270
Infrared radial velocities have been used to compute first orbits of theM giants in three southern S-type symbiotic systems. Of the three, Hen2-173 has the longest orbital period, 911 days, and also has anoncircular orbit with an eccentricity of 0.21. The large value of itsmass function suggests that Hen 2-173 may be an eclipsing system. For CLSco our spectroscopic orbital period of 626 days is essentiallyidentical to the previously determined light variability period of 625days, and we have adopted the latter. AS 270 has an orbital period ofsimilar length, 671 days, and both CL Sco and AS 270 have circularorbits. Only CL Sco has been extensively investigated previously, and wecompare our results with the conclusions of Kenyon & Webbink. Wealso have examined the period-eccentricity relation for 30 S-typesymbiotics. Circular orbits are found for 81% of the systems withorbital periods up to 800 days, while they occur for only 22% withperiods greater than 800 days. This distribution is quite unlike thatfor G and K giants; rather, it is similar to that for barium stars,another type of mass-transfer binary, which also consists of a late-typegiant and a white dwarf companion.
| Symbiotisch Veraenderliche von Typ Z And.
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| Binary Life after the AGB -- Towards a Unified Picture
We attempt to construct a unified evolutionary scheme that includespost-AGB systems, barium stars, symbiotics and related systems,explaining their similarities as well as their differences. Specialattention is given to the comparison of the barium pollution andsymbiotic phenomena. Finally, we outline a `transient torus'evolutionary scenario that makes use of the various observational andtheoretical hints and aims at explaining the observed characteristics ofthe relevant systems.
| Modeling Light Curves of Symbiotic Stars
Near-infrared light curves of some well-studied symbiotic stars show amodulation with half-orbital period as expected for an ellipsoidalvariability in the red giant. We present and discuss preliminary resultsof modeling of three symbiotic systems with such variability: RW Hya, SYMus and AR Pav.
| The Origin and Evolution of Symbiotic Binaries
Symbiotic binaries form a diverse class of interacting binaries that canbe used to constrain binary evolution theory. In this contribution wewill review some of the major unsolved issues concerning symbioticbinaries, in particular the origin of the orbital-period distribution ofS-type symbiotic binaries, their possible relation to a sub-class ofType Ia supernovae and the stability of mass transfer. We will thenpresent some recent observations of Mira suggesting that even very widesymbiotic experience a new form of Roche-lobe overflow, ``windRoche-lobe overflow''. We will present preliminary hydrodynamicalsimulations of this type of mass transfer and discuss its implicationsfor the shaping of (proto-)planetary nebulae, the origin of barium starsand potentially Type Ia supernova progenitors.
| Symbiotic Stars: Continually Embarrassing Binaries
This paper aims at presenting the state-of-the-art in understanding ofsymbiotic binaries. In particular, we discuss their basic parameters,the mechanisms of mass loss and accretion and the role of theseprocesses in the observed activity of symbiotic systems.
| Recent photometry of symbiotic stars
We present new photometric observations of 15 symbiotic stars coveringtheir last orbital cycle(s) from 2003.9 to 2007.2. We obtained our databy both classical photoelectric and CCD photometry. Main results are: EGAnd brightened by ˜ 0.3 mag in U from 2003. A ˜0.5 mag deepprimary minimum developed in the U light curve (LC) at the end of 2006.Z And continues its recent activity that began during the 2000 autumn. Anew small outburst started in summer of 2004 with a peak U magnitude of˜ 9.2. During the spring of 2006 the star entered a massiveoutburst. It reached its historical maximum at U˜ 8.0 in 2006July. AE Ara erupted in 2006 February with Δ m_vis ˜ 1.2 mag.BF Cyg entered a new active stage in 2006 August. A brightness maximum(U˜ 9.4) was measured during 2006 September. CH Cyg persists in aquiescent phase. During 2006 June-December a ˜ 2 mag decline inall colours was measured. CI Cyg started a new active phase during 2006May-June. After 31 years it erupted by about 2 mag in U. TX CVnmaintains a bright stage with U ˜ 10.5 from 2003. AG Dra entered anew major outburst in 2006 June. It reached its maximum at U ˜ 8.0in 2006 September. AR Pav persists at a low level of the activity. AGPeg's LC profile varies markedly during different orbital cycles. AX Percontinues its quiescent phase.The Tables of this article are available at the CDS viahttp://cdsarc.u-strasbg.fr/ftp/cats/J/other/AN/328.909
| Line formation regions of the UV spectrum of CI Cygni
Aims.They are the interpretation of the emission line formation regionsin CI Cygni Methods: .They involve theexamination of radial velocities and fluxes of ultraviolet emissionlines at different epochs, deduced from archival IUE and GRHS/HSTspectra. Results: .The line fluxes give electron densities andwere in addition used to calculate emission measures, suggesting lineformation in regions rather smaller than the binary separation.Examination of the radial velocities led to us to find a systematicredshift of the high ionization resonance lines with respect to theintercombination, and He II lines. Possible explanations of the redshiftand the high resolution GHRS C IV profile are discussed. We favour thatinvolving resonance line absorption by a circum-binary region mostprobably in an asymmetric wind interaction shell or in a wind from theaccretion disk.
| High-Ionization Forbidden Lines in the UV Spectrum of AG Draconis
High-ionization forbidden lines from Ca VII, Fe VII, Mg V, Mg VI, MgVII, and Si VII are found in recent Hubble Space Telescope STISultraviolet spectra of the symbiotic star AG Draconis. These specieshave ionization potentials between 99 and 205 eV, which are unexpecteddue to the high density (~1010 cm-3) of the AG Dranebula. The identification of the Mg VII λλ2510, 2629lines is the first in astrophysical or laboratory spectra, and revisedrest wavelengths are suggested from the STIS spectra. Plasma diagnosticsfrom Mg V-VII are applied, but do not provide a consistent constraint ontemperature or density. A density >=108 cm-3 isconfirmed, however. The lines show double-peaked profiles with widths~100-160 km s-1, suggestive of an origin in an accretiondisk. However, the line widths, if identified with motion in a Kepleriandisk, indicate radii much smaller than sizes inferred from the linefluxes themselves. The source of these high-ionization forbidden linesremains unidentified.
| UBV(RI)C photometric sequences for symbiotic stars. III
We present accurate UBV(RI)C photometric sequences andastrometric positions for a final set of 41 symbiotic stars. In asimilar manner to the 40 targets of Papers I and II, these sequencesextend over wide brightness and color ranges and are suited to coveringboth quiescence and outburst phases. They are intended to assist boththe CCD photometric monitoring of current variability and exploitationof old photographic plates from historical archives.
| Broad Hα wings from the optically thin stellar wind of the hot components in symbiotic binaries
Aims.To model broad Hα wings observed in symbiotic binaries by anoptically thin, bipolar stellar wind from their hot components as analternative to that considering the Raman scattering of Lyβ photonson atomic hydrogen. Methods: .Profile-fitting analysis. Comparisonof the observed broad Hα wings and their luminosity with thosepredicted by the model. Results: .Synthetic Hα profiles fitexcellently the observed wings for | Δ v | 200 kms-1 in our sample of 10 symbiotic stars during the quiescentas well as active phases. The wing profile formed in the stellar windcan be approximated by a function f(Δ v) ∝ Δv-2, which is of the same type as that arising from the Ramanscattering. Therefore it is not possible to distinguish between thesetwo processes only by modeling the line profile. Some observationalcharacteristics of the Hα-emission, its relationship with theemission measure of the symbiotic nebula and a steep radio spectrum at1.4-15 GHz suggest the ionized stellar wind from the hot component to bethe dominant source contributing to the Hα wings during activephases. The model corresponding mass-loss rates from the hot componentsare of a few × 10-8 Mȯ yr-1and of a few × (10-7 - 10-6)Mȯ yr-1 during quiescent and active phases,respectively.
| Population Synthesis for the Symbiotic Stars with Main-sequence Accretors
Using a population synthesis code, we have investigated the formation ofsymbiotic systems in which the hot component is a main-sequence starthat is accreting matter from the cool component via Roche lobe overflow(RLOF). The RLOF can be divided into two cases: dynamically unstable andstable. In the first case, the birthrate of symbiotic stars is 0.056yr-1 or 0.045 yr-1 depending on differentassumptions; in the stable RLOF case, it is 0.002 yr-1 or0.005 yr-1. The number of symbiotic stars with main-sequenceaccretors and unstable RLOF in our galaxy is about 5, that with stableRLOF is about 60 to 280. Comparison between our results with those ofYungelson et al. shows that symbiotic stars with MS accretors make onlya small contribution (lesssim8%) to the whole population of symbioticstars in the Galaxy.
| The Light Curves of Classical Symbiotic Stars
Results of both photoelectric and visual long-term monitoring ofselected classical symbiotic stars (CSS) are presented. We suggest aclassification in which the complex behavior in their light curves canbe basically divided into two groups: (i) periodic and (ii) non-periodicvariations. The possible nature of the observed variations is brieflydiscussed. Comparison of the visual magnitude estimates to thephotoelectric data is provided for individual cases.
| The nature of ultraviolet spectra of AG Pegasi and other symbiotic stars: locations, origins, and excitation mechanisms of emission lines
A detailed study of ultraviolet spectra of the symbiotic star AG Peg hasbeen undertaken to derive the atomic excitation mechanisms and origin offormation for the lines common in symbiotic systems. More than 600emission lines are observed in spectra from {IUE}, {HST} and {FUSE} ofwhich 585 are identified. Population mechanisms and origin of formationare given for a majority of those lines. Based on the understanding ofthe AG Peg spectra {IUE} data of 19 additional symbiotic stars areinvestigated and differences and similarities of their spectra arediscussed. Fe II fluorescence lines pumped by strong emission linesbetween 1000 and 2000 Å are observed in 13 of these systems. Someof the symbiotic systems belonging to the subclass symbiotic novae havemore than 100 Fe II fluorescence lines in the ultraviolet wavelengthregion. Forbidden lines are detected for 13 of the stars, mostly fromhighly-ionized spectra such as Ar V, Ne V and Mg V. Further, [Mg VI] and[Mg VII] lines are observed in a symbiotic star (AG Dra) for the firsttime. Five of the symbiotic stars have broad white-dwarf wind profiles({FWHM} > 400 km s-1) for a few lines in their spectra.The stars with no such broad lines can be divided into two similarlysized groups, one where all lines have FWHM less than 70 kms-1 and the other where one, a few or all of the broad({FWHM} > 400 km s-1) lines of AG Peg have an enhancedbroad wing (110-140 km s-1).
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Nonradial pulsations of the hot component of the symbiotic star CH Cyg during its active phase
We observed the symbiotic star CH Cyg with the 70 cm telescope of theCrimean Astrophysical Observatory in 1982, during its active phase, whenit was brighter than 6m in the V band. We simultaneously detected thecontinuum brightness at wavelengths of 3737, 5092, and 5500 Å witha time resolution of 20 s. We present light curves for these wavelengthsfor four nights between July 13 and August 22, demonstrating thecharacter of the star’s rapid variations. Variousfrequency-analysis methods were applied to the data series at 3737 and5092 Å, where the light from the hot component made an appreciablecontribution. For both series, we find some 20 similar frequenciescorresponding to periods ranging from 150 to 6000 s. Our estimates ofthe significance of the identified frequencies indicate that thesignificance level exceeds 3 σ in all cases, and is even higher inmost cases. For many of the oscillations, the amplitudes varied on timescales shorter than one day. Multiperiodicity and variable oscillationamplitudes are characteristic of nonradial pulsations. We conclude thatnonradial oscillations typical of a white dwarf’s g modes wereexcited during the symbiotic star’s state of highest activity.
| A ``Combination Nova'' Outburst in Z Andromedae: Nuclear Shell Burning Triggered by a Disk Instability
We describe observational evidence for a new kind of interacting binarystar outburst that involves both an accretion instability and anincrease in thermonuclear shell burning on the surface of an accretingwhite dwarf. We refer to this new type of eruption as a combinationnova. In late 2000, the prototypical symbiotic star Z Andromedaebrightened by roughly 2 mag in the optical. We observed the outburst inthe radio with the VLA and MERLIN, in the optical both photometricallyand spectroscopically, in the far-ultraviolet with FUSE, and in theX-rays with both Chandra and XMM-Newton. The 2 year long event had threedistinct stages. During the first stage, the optical rise closelyresembled an earlier, small outburst that was caused by an accretiondisk instability. In the second stage, the hot component ejected anoptically thick shell of material. In the third stage, the shell clearedto reveal a white dwarf whose luminosity remained on the order of104 Lsolar for approximately 1 yr. The eruptionwas thus too energetic to have been powered by accretion alone. Wepropose that the initial burst of accretion was large enough to triggerenhanced nuclear burning on the surface of the white dwarf and theejection of an optically thick shell of material. This outbursttherefore combined elements of both a dwarf nova and a classical nova.Our results have implications for the long-standing problem of producingshell flashes with short recurrence times on low-mass white dwarfs insymbiotic stars.
| Astrometric orbits of SB^9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to deriveastrometric orbital elements for spectroscopic binaries from the newlyreleased Ninth Catalogue of Spectroscopic Binary Orbits(SB^9). This endeavour is justified by the fact that (i) theastrometric orbital motion is often difficult to detect without theprior knowledge of the spectroscopic orbital elements, and (ii) suchknowledge was not available at the time of the construction of theHipparcos Catalogue for the spectroscopic binaries which were recentlyadded to the SB^9 catalogue. Among the 1374 binaries fromSB^9 which have an HIP entry (excluding binaries with visualcompanions, or DMSA/C in the Double and Multiple Stars Annex), 282 havedetectable orbital astrometric motion (at the 5% significance level).Among those, only 70 have astrometric orbital elements that are reliablydetermined (according to specific statistical tests), and for the firsttime for 20 systems. This represents a 8.5% increase of the number ofastrometric systems with known orbital elements (The Double and MultipleSystems Annex contains 235 of those DMSA/O systems). The detection ofthe astrometric orbital motion when the Hipparcos IAD are supplementedby the spectroscopic orbital elements is close to 100% for binaries withonly one visible component, provided that the period is in the 50-1000 drange and the parallax is >5 mas. This result is an interestingtestbed to guide the choice of algorithms and statistical tests to beused in the search for astrometric binaries during the forthcoming ESAGaia mission. Finally, orbital inclinations provided by the presentanalysis have been used to derive several astrophysical quantities. Forinstance, 29 among the 70 systems with reliable astrometric orbitalelements involve main sequence stars for which the companion mass couldbe derived. Some interesting conclusions may be drawn from this new setof stellar masses, like the enigmatic nature of the companion to theHyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but thecompanion remains elusive.
| Disentangling the composite continuum of symbiotic binaries. I. S-type systems
We describe a method of disentangling the composite, 0.12-5 μmcontinuum of symbiotic binaries. The observed SED is determined by theIUE/HST archival spectra and flux-points corresponding to the opticalUBVRI and infrared JHKLM photometric measurements. The modeled SED isgiven by superposition of fluxes from the cool giant, hot stellar sourceand nebula including the effect of the Rayleigh scattering process andconsidering influence of the iron curtain absorptions. We applied thismethod to 21 S-type symbiotic stars during quiescence, activity andeclipses. We isolated four main components of radiation and determinedtheir properties. (i) Stellar radiation from the giant corresponds to aunique luminosity class - normal giants. Characteristic luminosities are1600 ± 200 and 290 ± 30 Lȯ for red andyellow giants, respectively in our sample of objects. (ii) Hot objectradiation during quiescence consists of the nebular and stellarcomponent. The former radiates at a mean electron temperature of 19 000K and its amount of emission suggests a mass-loss rate from giants viathe wind at dot MW = a few × 10-7Mȯ yr-1. Radiation of the latter conformswell with that of a black-body photosphere at a characteristictemperature of 105 000 K. The corresponding effective radii are a factorof 10 larger than those of white dwarfs, which thus precludes observingthe accretor's surface. Extreme cases of AX Per and V443 Her, for whichthe hot star temperature from the fit is not capable of producing thenebular emission, signal a disk-like structure of the hot stellar sourceeven during quiescence. (iii) Hot object radiation during activityconsists of three components - the stellar and the low- andhigh-temperature nebular radiation. The stellar radiation satisfies thatof a black-body photosphere at a low characteristic temperature of 22000 K (we call it the 1st type of outbursts) or at a very highcharacteristic temperature of ≈165 000 K (2nd type of outbursts). Allthe active objects with a high orbital inclination show features of the1st-type of outbursts (here Z And, AE Ara, CD-43circ14304, TXCVn, BF Cyg, CH Cyg, CI Cyg, AR Pav, AX Per), while AG Dra representsthe 2nd-type. The presence of a two-temperature type of UV spectrum andan enlargement of effective radii of the stellar source by a factor of 10 with respect to the quiescent values during the 1st-type of outburstsuggest an expansion of an optically thick medium at the orbital planein the form of a disk. The low-temperature nebula radiates at a meanelectron temperature of 14 000 K and is subject to eclipses, while thehigh-temperature nebula, which is seen during eclipses as the onlycomponent, is characterized by Te > 30 000 K. Radiativeand geometric properties of the main sources of radiation allowed us toreconstruct a basic structure of the hot object during the 1st-type ofoutburst. There is an edge-on disk around the accretor. Its outer flaredrim represents a warm pseudophotosphere of the hot stellar source, whoseradiation is Rayleigh attenuated and affected by the iron curtainabsorptions in the neutral gas concentrated at the orbital plane. Thelow-temperature nebula is placed just above/below the disk with aconcentration at its edge as to be subject to eclipses and to“see” well the central ionizing source. High above/below theorbital plane, there is a hot nebular emitting region.
| The spectroscopic orbits and other parameters of the symbiotic binary FN Sgr
We present a study of the eclipsing symbiotic binary FN Sgr with aperiod of 568.3 days determined photometrically and confirmedspectroscopically. The hot component underwent a 2.5 mag eruptioncovered by most of our spectroscopic observations. In particular, wehave determined for the first time spectroscopic orbits based on theradial velocity curves for both components. A set of blue absorptionlines resembling an A-F type star is present in all our spectra and theyseem to be associated with the hot component. Based on the light curve,we derive the red giant's radius (140~ Rȯ) and theorbital inclination (i = 80°). We find that FN Sgr is similar toother S-type symbiotic binaries, composed by an M 5-type giant (M_g=1.5Mȯ) and a hot white dwarf (Mh = 0.7~Mȯ, R_h= 0.2~ Rȯ) with a binaryseparation of ~1.6 AU. The red giant is just filling its Roche lobe anda geometrically and optically thick accretion disk is likely to bepresent around the low-mass accretor. The evolution of Th andLh along the active phase argues in favour of accretion diskinstabilities similar to those of Z And. We have also studied spectralchanges and photometric variations as a function of both the hotcomponent activity and the orbital motion.
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