We present preliminary statistical results of an ongoing polarimetric survey of symbiotics. So far, 38% of the known symbiotics have had their polarization measured at least once. About 40% of S and D types show some intrinsic polarization while only one of the five measured D’ types is polarized. R Aqr is still the only symbiotic with strong, variable polarization.

A model of a symbiotic system is outlined. We present evidence that the main ionization mechanism responsible for the symbiotic nebula is radiative and not collisional. For calculating fractional ionization throughout the emission region we take either a blackbody source or an accreting system. If the ionizing radiation is due to accretion, the boundary layer between accreting star and disc provides the bulk of the ionizing photons. The possibility for distinguishing between different ionization sources are briefly discussed. C/N and O/N abundance ratios, calculated with the outlined ionization model places HM Sge, RS Oph, and T CrB among the novae and not among the symbiotic stars.

The diagnositic possibilities for identifying the ionizing source in symbiotic systems are explored. As possible sources we consider hot blackbodies and accretion discs. It turns out that main sequence accretors and hot blackbodies may have the same appearance in both, emission line and continuum flux distribution. However, UV continuum indices of models containing an accretion disc around a white dwarf are confined to a very small region, separated from main sequence accretors and blackbodies. Furthermore, if symbiotic systems containing a white dwarf accretor exist, they might be recognizable by strong emission in Fe X λ6374.

A 2-dimensional classification system for symbiotic stars based on the 5007/4861 and 3727/5007 line ratios adapted from the method of Baldwin et al.(1981) is presented. It is shown that a simple measurement suffices to classify stars and no reddening has to be taken into account. Symbiotics fall into 2 distinct classes in the line ratio plot. The dusty and S types. Neither type coincides with either the planetary nebulae or the HII region locus. The highest excitation is found for the dusty types. The two dimensional distribution can be interpreted as evolutionary, the more evolved Mira containing symbiotics lying closer to the PN locus. The fact that there are few D, D’ type symbiotics would indicate that the phase between Mira and PN is short.

A compact star in a detached binary system can accrete the matter from the stellar wind of the companion. In this case a more or less radial accretion flow is formed. By analogy to stellar winds from early type stars it is usually believed that the accretion of this sort should produce inverse P-Cygni profiles in resonance lines. However, there are physical differences between the wind and the radial accretion which can alter the outgoing profile significantly.

The matter outflowing from an early type star cools off very fast due to the adiabatic expansion. However, it remains highly ionized as the quickly decreasing density does not allow it to recombine. Therefore the principal mechanism for the resonance line formation here is the scattering of the stellar continuum photons in the wind.

The physics of colliding winds in symbiotic systems is reviewed. The theoretical predictions are compared with observational data of symbiotic novae, in particular the recently erupted system HM Sge. It is suggested that the spectral behaviour of HM Sge from X-ray to radio can be explained by the colliding winds process.

We give an overview over the theory of geometrically thin α-accretion disks: further we introduce the two different proposed mechanisms that can cause outbursts of accretion disks: and finally we compare the results of these models applied to symbiotic stars (=SS).

Recent calculations have demonstrated that accretion from a stellar wind is very probably unsteady. The average rate of accretion of angular momentum is lower by about a factor 5 than the rate at which angular momentum is deposited into the Bondi-Hoyle accretion cylinder. This makes disk formation from wind accretion very difficult, in particular in the case of massive x-ray binaries. A combination of x-ray, uv and optical observations of symbiotic and related systems, as well as spin-up information on x-ray binaries, can be used to determine whether an accretion disk does form. Such observations can provide us with valuable information on the process of accretion from an inhomogeneous medium.

This paper reviews the basic physics of thermonuclear runaways on the surfaces of accreting white dwarf stars, with a special emphasis on understanding the evolution of symbiotic novae.

The most commonly advocated models for explaining the outburst of symbiotic systems are thermonuclear runaways, modulated mass accretion through a disc, or accretion disc instabilities. We show that the onset of a strong stellar wind on the cool component can also produce the observed increase in luminosity.

Preliminary results of 1–D fully hydrodynamic numerical calculations including accretion and mass loss on a 1M⊙ hot white dwarf are presented. For the case of Ṁ = 10−8M⊙/yr and Teff = 125000°K we get flashes of duration of ∼ 30 – 40 yrs which reccur every ∼ 1500 years. During each flash ∼ 6x10−6M⊙ is lost (∼ 40% of the accreted envelope.

The drag luminosity produced by a secondary embedded in a nova envelope is calculated and it is assumed to liberate gravitational energy. The results of a preliminary calculation show that the time for envelope ejection and therefore for the light decay varies sharply with the binary distance. It would be considerably reduced if the binary distance is a < 2 RO. It is suggested that this is the main difference between the thermonuclear runaway of symbiotic stars and nova systems, instead of other physical parameters.

This paper reviews the general discussion on the physics of symbiotic stars.

Z Andromedae, often considered as the prototype of symbiotic stars, experimented, after several years of quiescence, a small outburst in March-April 1984, followed by a larger one in September-October 1985. The imminence of a new activity phase was predicted by Viotti et al. (1982). Z And is, together with AG Draconis, the only symbiotic star observed both during quiescence and activity with the IUE satellite. The early photometric and spectroscopic history of Z And has been recently reviewed by Kenyon (1986).

The behaviour of Z And in the ultraviolet during quiescence has been studied by Fernåndez-Castro et al. (1988), on the basis of data obtained by the International Ultraviolet Explorer from 1978 to 1982. In that period of time, the UV continuum and the emission line fluxes varied quasiperidically with a period of about two years, in phase with the Hα variability found by Altamore et al. (1979), with the UBV photometry by Belyakina (1985) and also in agreement with the ephemeris given by Kenyon and Webbink (1984). The electron density derived from Si III/C III] flux ratio, also varied in phase with the UV flux. In particular, a correlation was found between the electron density variations, and the UV continuum flux at 2900 A. This fact, together with the presence of a Balmer jump in emission, indicates that the principal contributor at those wavelengths is nebular emission, mainly free-bound transitions.

The observed properties of CI Cyg are reviewed. Spectroscopic and photometric changes due to orbital effects as well as intrinsic variability of the components are discussed.

Infrared spectroscopic observations of CI Cyg in eclipse show a P Cygni profile for He I 10830 Å, with a wind velocity of the order of 150 km/s, not seen in hydrogen Brackett and Paschen emission lines, which are single peaked.

It has been shown that the cold component of CI Cyg eclipsing binary system is a variable star. The amplitude of its light variations is close to with the time interval of 40–60 days.This star being the red giant M4 can be attributed as SR type variability. The light variations is caused by its temperature variations.

AG Dra is probably a metal poor symbiotic binary of the galactic halo. The luminosity of the cool component is uncertain, which contributes to doubts about the correct model. Behaviour in activity is non classical with high ionization emission lines still having been seen then in the spectrum. This may perhaps most easily be explained by a weak thermonuclear event.

After a long quiet phase AG Dra underwent an outstandingly active phase with two outbursts in 1980 Nov. and 1981 Nov.(Viotti et al, 1984). Since then a new quiet phase has followed. In this work we analyse two spectra of AG Dra, of which, one was taken in 1981 by C.C. Huang at the Haute-Provence Observatory using the Marly spectrograph with a dispersion of 80 A/mm at the 1.2 m telescope, the other was obtained by Dr Y.Andrillat in 1985 with the same instrument.

Figure 1 shows the spectral variations of AG Dra between 1981 and 1985. The main features of the emission line spectrum are not much different between the two spectra, except that in 1985 there was a new wide weak emission line at 3488 A possibly due to FeII. There were a lot of strong emission lines due to H, HeI, HeII and 0III in both spectra. The spectrum of the late-type component was much more obvious in 1985 than in 1981. In 1985 the lines of CaII K, CaIλ4227 and the G-band of CH were quite strong. In addition we measured a lot of absorption lines due to FeI, SrII and TiII on the 1985 plate. The Balmer continuum emission and the blue contiuum were enhanced in 1981. On the 1981 plate the stellar spectrum in ultraviolet can be traced beyond 3200 A and the blue continuum heavily veiled the spectrum of the cool component. In 1985 they were much weaker.

The results of multi-frequency observations of CH CYG during the last outburst are reviewed, and the time-sequence of the variations related to the jets formation is discussed.