The Nature of Classical Symbiotic Stars

The results of the optical and infrared observations of classical symbiotic stars Z and, CI Cyg, BF Cyg, AG Dra, AX Per, V443 Her, and YY Her are summarized. It is shown that the hot component of most classical symbiotic stars is a hot subdwarf and not a Main-Sequence star with an accretion disc. The energy source of its outbursts is the gravitational energy of the matter accreted from the cool component's surface. The cool component is a red giant filling the Roche lobe and having class II luminosity. In the intervals between outbursts the hot component's luminosity may be determined by its own energy sources. It is probable that among classical symbiotic stars there are-in an insignificant quantity-systems in which the hot component is a Main-Sequence star with an accretion disc. In such systems eclipses of the hot source of radiation by the red giant must without fail occur and the hot component must be a yellow or red dwarf. The transition from a symbiotic nova (V1016 Cyg, HM Sge, and RR Tel) to a classical symbiotic nova takes place at the moment when the cool component's size is approaching the size of the Roche lobe, resulting in a sharp increase of the accretion rate of its matter onto the hot component. The nonstationarity of this process leads to the appearance of nova-like outbursts on classical symbiotic stars' light curves.