The investigation of the atmospheric structure and chemical composition of CH stars. These are giant G5-K5 stars with enhanced CH bands and are characterized with weak metal lines, but heavy elements enhanced. The luminocities of the CH stars in our Galaxy is less than the luminocity of AGB but of AGB luminocities in the LMC. Most of the sub-AGB carbon stars (BaII stars and sub-giant CH stars) are believed to gain their high carbon and heavy elements abundances in the mass-transfer from the more massive component which has produced these elements on AGB. Our results indicate that cooler CH stars may be the intrinsic carbon stars and may be similar to the carbon stars found in the dwarf galaxies and in the Magellanic Clouds. The sample is, however, quite small and we are trying to enlarge it (T.Kipper in collaboration with U.G.Jorgensen (NBI, Denmark)).
One of our objects is FG Sagittae, the central star of the planetary nebula He 1-5. FG Sge is a unique object in the sense that for this star we have the direct evidences of stellar evolution in the time scale comparable with the human lifetime. From the end of the last century it has evolved across the HR diagram changing from a normal hot giant to a late spectral type star with the marked changes in its surface chemical composition. It is believed that FG Sge has suffered the "final" He-flash after departing from the ABG on its way to WD-s. In the recent years FG Sge has been very active resembling now the R CrB stars with outbursts of carbon dust. We are spectroscopically following the changes in this very interesting star (T.Kipper in collaboration with V.G.Klochkova (SAO, Russia)).
In 1996 a new object which has possibly suffered the "final" He-flash was discovered by I.Sakurai, Japan. We started the spectroscopic studies of this object also (T.Kipper and V.G.Klochkova). The first results show that the Sakurai's object is hydrogen deficient and probably carbon enriched. The spectrum resembles closely that of R CrB stars. No carbon dust shells are yet found but the object is the central star of a planetary nebula as is FG Sge.
In some WR stars the optical depth of the outer layers above the interaction zone is large enough so that after some distance the extra ionization zone ends and normal (low) ionization state is restored. Strong (about three times the mean) variations of the radio fluxes of WR stars may occur if the interaction zone is unstable. Strong variations of radio fluxes in the cm-region are observed in the case of LSS 4065 (WN7) and HD 68273 (WC8+O9I) and probably also in the case of HD 93131 (WN7+OB) and HD 92740 (WN7+OB).
The important question concerning the mass loss rate determinations for WR stars is whether the enhancement factor for N**2 processes due to clumping is greater than unity in the far wind regions where radio fluxes in the cm-region are effectively formed? In recent studies (e.g. Moffat & Robert 1994: ApJ 421, 310) it has been concluded that the flux enhancement factors due to clumping must be greater than unity for radio emission and that the neglect of this enhancement of radio fluxes may lead to overestimates of mass loss rates for WR stars (about 2-5 times). The key object for making such conclusions is the eclipsing binary system V444 Cyg = HD 193576 (WN5+O6V). This system is usually regarded to be a member of the open cluster Berkeley 86. The comparison of the parameters of Be 86 and V444 Cyg leads to the conclusion that this is not the case. T.Nugis found that the distance to V444 Cyg is 1.15+-0.13 kpc (the observed value of the radius of the O6V component was used in deriving this estimate). The mass loss rate of the WN 5 component in V444 Cyg as determined from the observed value of the radio flux at 4.9 GHz is (0.95+-0.35)x10**(-5) M(solar)/year when using the standard formula and the new estimate for the distance. This mass loss rate is in accord with the estimates obtained from the period change rate and from the polarimetric studies. The flux enhancement factors (due to clumping) for radio emission of WR stars ought to be about unity but for getting reliable mass loss rates from radio fluxes, we need to know the true ionization state in the effective radio emission zone.
T.Nugis in collaboration with A.J.Willis and P.A.Crowther (University College London) determined the clumping-corrected ``radio'' mass loss rates for some tens of WR stars using spectral indices in the IR/mm/cm spectral ranges and line fluxes for finding the state of ionization in the effective radio emission zone.
K.Annuk is continuing spectroscopic observations of some long period WR stars (HD 193793, HD 192641).
The peculiar symbiotic system CH Cygni has been one of the "pet symbiotics" of our Observatory for about thirty years. At present, spectroscopic and photometric observations of CH Cygni are carried out continuously. From recent developments, the most interesting was an unprecedented fading of CH Cygni down to 10th stellar magnitude in June-July 1996.
The long-period binary star WY Geminorum, containing a M2 supergiant, is also under continuous study. This star belongs to the VV Cephei type stars. The prototype star VV Cephei itself is undergoing an eclipse of the hot component in 1997-1998, and we have started spectroscopic monitoring of the eclipse.
Photometric observations of the unusual triple star CQ Draconis are carried out occasionally, as well as spectroscopic observations of several other symbiotic-like binaries, e.g. AX Monocerotis (studied by post-graduate student Alar Puss), EG Andromedae, BU and TV Geminorum, V694 Monocerotis=MWC 560 et al.
The eclipsing binary RX Cas consists of orange K-giant which overfills it's Roche' lobe and the matter overflow forms an accretion disk around the hot primary star of yet unknown nature. The spectroscopic monitoring (started in 1995 by I.Kolka) of this binary system would reveal physical parameters of the accretion disk and the nature of the primary component.