SAAO photometry of RX Pup at 3.5 μ between 1972 and 1981 shows a very well defined period of 580 days with an amplitude of 0.35 mag, which is attributed to a Mira variable component. The infrared variability of RX Pup was demonstrated by Feast, Robertson and Catchpole (1977), who also pointed out that the infrared colours were indicative of the presence of a Mira variable with a dust excess. Further evidence for the Mira comes from the observations of H2O in the infrared spectrum by Barton, Phillips and Allen (1979).

HM Sagittae is one of the most unusual objects in the Galaxy for it displays activity in every spectral band from x-ray to radio. Its present variable-star designation was given after the discovery of its optical brightening from 16m to 12m between April and September 1975 (Dokuchaeva and Balazs 1976). It was soon found to have a rich emissionline spectrum similar to that of a planetary nebula (Stover and Sivertsen 1977). Post-brightening monitoring of the object by Ciatti, Mammano and Vittone (1977, 1978) found the B and V magnitudes to be variable with amplitudes of at least one magnitude. Evidence for increasing excitation was found by Ciatti, Mammano and Vittone (1979) with Hell 4686 emerging in October 1978. Wolf-Rayet features of velocities up to 2000 km s-1 have also been seen (Belyakina, Gershberg and Shakhovskaya 1978, 1979; Brown et al. 1978; Ciatti et al. 1978; Wallerstein 1978; Allen 1980; Andrillat and Swings 1982). Analysis of the forbidden line ratios gives an estimated nebula density of 106-107 cm-3 and a gas temperature >104K (Ciatti et al. 1977; Arkhipova and Dokuchaeva 1978; Davidson, Humphreys and Merrill 1979; Arkhipova, Dokuchaeva and Esipov 1979).

We have observed the ultraviolet spectrum of HM Sge with the International Ultraviolet Explorer during 1978-1980.

In 1948 the South African amateur astronomers de Kock and Kirchhoff were the first to notice that RR Tel had brightened to 7th magnitude. Subsequent examination of the Harvard Patrol plates showed that it had in fact reached a maximum in 1944 after rapidly brightening 7 magnitudes (Mayall, 1949). Prior to 1944 the object showed variations with a period of 387 days and an amplitude of up to 2 magnitudes in the blue. After the outburst the spectrum evolved in a manner characteristic of very slow novae: an F5 supergiant absorption spectrum gave way to strong permitted and forbidden emission lines. The ionization levels characterising the emission increased with time. The spectral development has been extensively studied and has been well summarised for the period up to 1973 in the Thackeray's (1977) monograph on the subject.

The ultraviolet spectrum of RR Tel was extensively studied with the IUE satellite since 1978 in both the high and low resolution modes. A comprehensive study of these observations was made by Penston et al.(l98l) who measured more than 400 emission lines. As it is clearly shown in figure 1, the UV spectrum of this symbiotic star is very rich in emission lines. Like in the optical spectrum (Thackeray 1977), the UV spectrum presents emission lines of ions belonging to a wide range of ionization energies, from neutral up to four and five times ionized species (OV, MgV, CaVI, etc.). Permitted, intercombination and forbidden transitions were found which may allow a diagnosis of the physical conditions of the emitting regions. Electron temperatures of 1.2-1.9 104°K and densities of 106-108 cm-3 were derived by Penston et al.

V 4049 Sgr was detected by the authors as a possible symbiotic star on Curtis Schmidt objective prism plates. This star is Nova Sgr 1978 and was first discovered by Stenholm and Lindstrom. When observed in 1979 With SIT specgrograph on CTIO 4 m telescope the outstanding features were the presence of very strong lines of [FeVII] chiefly at 6085 A where its flux relative to H α is 1.6. Details have been published in Pub. astr. Soc. Pacific, 93, 470 (1981).

U, B, V, J, H, K photometry, polarimatry and spectroscopy of this peculiar object was made from August 1979 to December 1980. During 1979 it was at a phase of stable maximum brightness with variations of less than 0m.2 in magnitude and less than 0m.1 in colour. A change occurred at the beginning of 1980;the object was 0m.5 fainter in mid March and then faded rapidly. During the rapid fading the object became redder in U-B and B-V, and there was an intrinsic polarization of about 1%. At minimum in September 1980, B-V become even bluer than at maximum, while the 1.2-2μ flux was twice lower in October 1980 than in April 1979. At maximum the spectrum was of type F; the Ball lines indicating a luminosity higher than that of a normal supergiant.

Symbiotic stars have been studied for many years. But the common determination of the term “symbiotic star” does not exist even now. Merrill (1958) introduced this term in order to emphasize unusual spectral features – absorption TiO bands and emission lines belonging to highly ionized ions.

If we look on the eruptive stars spectra we well see absorption and emission features in the spectra of many stars which we cannot certainly consider as symbiotic stars. For example the U Gem type stars have absorption and emission features in their spectra. A similar situation exists in the case of the old novae. The T Tau type stars have many emission lines, and their spectral type corresponds to G-K. Even the long period variables have some emission lines though their spectral type is M. On the other hand, there are also the BQ[]type stars which are characterized by the presence in their optical spectrum of forbidden emission lines and of a rather hot absorption spectrum.

Symbiotic stars have become an important testing ground of various theories of binary star evolution. Several physically different models can explain them, but in each case certain fairly restrictive conditions must be met, so if we manage to identify a definite object with a model, it will tell us a lot about the structure and evolutionary stage of the stars involved. I envisage at least three models that can give us a symbiotic object: I have called them, respectively, the PN symbiotic, the Algol symbiotic, and the novalike symbiotic. Their properties are briefly discussed. The most promising model is one of a binary system in the second stage of mass transfer, actually at the beginning of it: The cool component is a red giant ascending the asymptotic branch, expanding but not yet filling its critical lobe. The hot star is a subdwarf located in the same region of the Hertzsprung-Russell diagram as the central stars of planetary nebulae. It may be closely related to them, or it may be a helium star, actually a remnant of an Algol primary which underwent the first stage of mass transfer. In these cases, accretion on this star may not play a significant role (PN symbiotic).

Different single and binary models of symbiotic stars are examined. Single star models encounter a number of problems, and binary models are probable. There are however difficulties in the interpretation of radial velocities. Accretion disks play a role in some cases, but winds especially from the cool component must be taken into account in realistic models. There is some evidence of excess heating of the outer layers of the cool component. Outbursts may be related to sudden changes in the characteristics of the cool star wind.

Observations of symbiotic stars in the far UV have provided important information on the nature of these objects. The canonical spectrum of a symbiotic star, e.g. RW Hya, Z And, AG Peg, is dominated by strong allowed and semiforbidden lines of a variety of at least twice ionized elements. Weaker emission from neutral and singly ionized species is also present. The Mg II doublet is usually very strong and may be associated with the M giant primary. A continuum may or may not be present in the 1200 - 2000 A range but is generally present in the range 2000 - 3200 A range, the latter arising from free-free and bound-free emission in the same nebula that is responsible for the UV line emission (CI Cyg, RW Hya, RX Pup). The suspected hot subdwarf continuum is seen in some cases in the range 1200 - 2000 A (RW Hya, AG Peg, SY Mus).

The evolutionary relations between symbiotic stars and cataclysmic variables are presented. The symbiotic stars are assumed to be long period detached binaries containing a carbon-oxygen degenerate primary and a red giant losing its mass through a spherically symmetric wind. Such systems can be obtained in Case C evolution, provided a common envelope during a rapid mass transfer phase was not formed. The same way recurrent novae containing a red giant as a secondary component may be produced. The factors influencing the differences between symbiotic stars and nova-type stars are discussed.

We discuss conditions necessary for appearance and discovery of the symbiotic star phenomenon within the model of a binary consisting of a red (super)giant 3 Mʘ not filling the Roche lobe and of an accreting hot degenerate CO-dwarf 0.8 Mʘ. Within this model “classical” symbiotic stars may exist only within a narrow region of mass accretion rates and separations of components: 10-7 ≲ Ṁ ≲ 3.10-7 Mʘ/y and 3.1013 ≲ a ≲ 2.1014 cm. The evolutionary status of symbiotic stars and related objects and the mechanisms of their variability are discussed.