Published online by Cambridge University Press: 12 April 2016
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.