Symbiotic stars. Evolutionary considerations.

Models of two kinds of symbiotic stars are proposed. It is assumed that these stars are binaries and that the degenerate dwarf accretes matter from a wind of the secondary component. In type I symbiotic stars (those of Boyarchuk) the luminosity is produced in a stably burning hydrogen shell. Variations of the accretion rate in a range bounded by two critical values can cause large variations in radius and effective temperature at almost constant bolometric luminosity. In type II symbiotic stars (e.g., V 1016 Cyg) the accretion rate is smaller than the lower critical value. In that case the hydrogen burning proceeds through shell flashes. It is suggested that type II symbiotic stars may be related to slow recurrent novae.