I give a very brief summary of the state of our knowledge of the symbiotic stars, together with some of my hopes for how the field will develop.

This paper reviews the discovery of symbiotic stars, and introduces multifrequency observations made during the past two decades.

The light variability of V641 Cas was discovered by Guinan et al. (1982). Spectral investigations were made earlier by Barbier(1971,1975), wich classified it as a VV Cep-type star and suggested that the V641 Cas is a binary system consisting of M3Ia supergiant and a B type star. Shaw and Guinan(1985) estimated the spectral class of the companion as B2.5. Recently Guinan et al. (1986) reported about semiregular changes with maximal amplitude of about 1.0mduring 1979–1986. Slow variations in the brightness with a mean period 310d and variable amplitude of 0.3m–0.9m ph are established by Berthold(1983).

The peculiar red giant HD 59643 (C6,2) has a composite spectrum like a symbiotic star’s except for the absence of highly ionized species. UV observations show a highly variable hot continuum with superimposed absorptions of Fe II and the common emission lines of C IV, Si III], C III], C II], and Mg II. These Fe II absorptions, which are practically ubiquitous in interacting binaries, are an effective diagnostic of turbulence and distribution of scattering gas, even at low resolution.

For the past 6 years we have been measuring the radial velocity of the M-giant component of Symbiotics Stars with high dispersion echelle spectrographs. Because these velocities are based on the absorption line spectrum of the M-star, very reliable orbital radial velocity curves can be constructed. The orbital solutions derived from these velocity curves allow the approximate size of the binary to be determined, and also provide insight into the origin of the photometric variations seen in these stars. For most of the stars, the photometric variations are due to eclipse or reflection effects (two notable exceptions are Z And and RW Hya). Stars which may be near to filling their Roche lobes are TX CVn, Z And, EG And, UV Aur, and CI Cyg. Stars which do not appear to fill their Roche lobes are AG Dra, RW Hya, and AG Peg.

The optical light curves of the three symbiotic stars AG Dra, AX Per and AG Peg, in the U, B and V photometric bands, in the quiescent states of these stars, are interpreted on the basis of a “reflection” model for the observed light variations. A least squares fit of a numerical model that is constructed to the observed magnitudes of the stars yields the value of few of the orbital elements of these binary systems. Some predictions are made on the basis of the model and its parameter values about the detailed structure of the light curves of the three stars, and of the expected structure of radial velocity curves of the cool components in these systems.

In 1975 Webster and Allen (1975) divided all symbiotic stars into two groups-those in which the 1-4μm continuum show only the presence of a cool star (type S),and those in which dust emission dominates (type D). With the exception of some of yellow symbiotic stars, the dust presence in others correlates with the spectral type of their cool components. That is why one can say that S-type symbiotics contain red giants with spectral type earlier than M6-M7.

At the IAU Colloq. N 7Q Allen (1982) noted that it is difficult to escape the conclusion that symbiotic stars contain normal cool giants. Nowadays it is certaiu to be correct because the modern observations of S-type symbiotic stars have not yet discovered any specific distinctions between their cool stellar components and normal red giants. At the same time it should be noted that some of these, for example, Z And, CI Cyg may be interacting binaries in which the cool component apparently fields its Roche lobe and unstable accretion of gas from the red giant onto its hot companion leads to the out bursts of the latter (Kenyon and Webbink 1984; Yudin 1987).

Absolute continuum energy distributions for 1200 A to 100μ have been derived for eight quiescent symbiotics. These data are a combination of contemporaneous satellite and ground-based observations, including low resolution IUE and optical spectrophotometry, broadband JKLM infrared photometry and IRAS Point Source Catalogue fluxes. The resulting multifrequency energy distributions represent a merged, uniformly calibrated set of observations and provide a unique opportunity for comparison with quiescent theoretical models.

Symbiotic Miras are identified by their infrared characteristics. It is shown how an understanding of the evolutionary position of normal Mira variables together with the empirically established period luminosity relation can be used to derive various physical parameters for similar objects in symbiotic systems. The pulsation periods of symbiotic Miras measured so far fall between 280 and 580 days, their ages must be in the 5–10 Gyr range while main sequence masses of the order 1 to 1.5 M⊙ are indicated. The obscuration events seen in the IR light curves of several symbiotic Miras are highlighted as potentially important and possible causes are discussed.

This review describes recent conclusions about the physical environment of red giant and supergiant stars. This includes coronae, chromospheres, dust formation and stellar winds. This knowledge can provide the boundary conditions for considering what role such objects play as members of binary star systems, where tidal forces and companion behavior alter observed characteristics.

Symbiotic stars are strongly believed to be binary systems - a hot component presumably a white dwarf ionizing and exciting gas from a cool primary star undergoing mass-loss. Silicate dust temperatures for six symbiotic stars namely CH Cyg, UV Aur, HM Sge, RX Pup, V1016 Cyg and RR Tel are determined for the first time using IRAS-LRS data. Some of these stars showed an outburst activity which was caused probably due to hydrogen shell flash in the secondary white dwarf as a result of mass transfer from the primary M giant; while others had been relatively quiescent. All the stars showed indications of the presence of silicate dust grains except perhaps UV Aur. The dust temperatures in the symbiotic stars which had in the past an outburst activity are in general much smaller (by 400 K) than those in the symbiotics which had been quiescent and not shown any nova-like activity. Also there is an indication from our analysis that in the case of the former the dust masses are larger and the shells are spatially more extended than in the case of the latter. These results are given in detail in Table 1. Our results also indicate that the grain sizes could be larger, the earlier the outburst was, as revealed by the relative strengths of the 10 and 18 μ features in the case of the symbiotic novae. Our results support the hypothesis that during an outburst there would be a common envelope of dust blown farther and farther away by the nova wind resulting in the dust parameters described in the paper; while in the case of the quiescent symbiotics, the Mira dust envelopes get constantly heated up by the hot companion star.

We have undertaken an extensive program to investigate atmospheric shock dynamics in Mira variables through monitoring the MgII emission strengths versus phase. Theoretical calculations of Bowen (1987), which represent the structural variations in the extended atmosphere of a pulsating Mira, are presented in addition to the MgII observations. The model calculations generate density, temperature and velocity structures as determined by stellar parameters and by interactions of the driving amplitude, dust opacity and thermal relaxation processes. The main diagnostic provided by the models is the total emitted shock luminosity versus phase, which can be compared to the observed MgII emission. Models and observations are presented for three Miras: S Car, R Car and T Cep.

Radio thermal bremsstrahlung emission has now been detected at centimeter wavelengths from about thirty symbiotic stars. These data combined with optical and IR data show that the radio emission in most systems may be understood in terms of the ionization of part of the wind of a red giant by a hot companion. The radio properties of symbiotic stars are reviewed and the agreement with and the limitations of this picture are examined.

A brief review of radio observations of individual symbiotic stars is presented, with emphasis on radio imaging of the circumstellar nebulae. The ionized nebulae are catagorized into two types: outburst ejecta and stellar winds. Among the ejecta-type there is a strong tendency for bipolar or jet morphology. In the case of the quiescent, stellar wind emitter, a few very wide binary systems (eg. Hl-36) offer the potential of directly viewing the effects of the binary interaction.

Spectroscopic observations of high spatial and high spectral resolution indicate that mass flow from symbiotic stars generally exhibits a bipolar pattern. Besides the polar features moving at velocities up to several 100 km/s, equatorial structures of much lower expansion rate are present in some cases. Mostly, the high-velocity components appear to be highly collimated and hence can be considered as “jets”. The jets probably originate from an accretion disk within a binary star where mass from a late-type giant is accreted by a compact companion.

A review of some physical mechanisms that may give rise to detectable optical polarization in symbiotic stars is presented, emphasizing the ability of polarimetry in studying the evolution of dust envelopes, the atmospheric structure of the cool component and the geometry of these systems. A brief summary of the techniques is also given. General polarimetric properties of symbiotics as well as data for a few specific systems are discussed. Proposed models for symbiotics that include the prediction of their polarimetric properties are highly encouraged. Specific suggestions for future work from the ultraviolet through the infrared and which explore the potential of the technique are advanced.

In an effort to bring out the variations in the polarization parameters of symbiotic stars that result from their binary character, polarimetry in R, I IR bands was initiated at the Byurakan station of the Leningrad University Observatory in 1974. In addition, U, B, V measurements for some stars have been made. Three telescopes have been used, with mirror apertures of 48, 48, 62 cm.