AbstractSymbiotic 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).
Molecular Envelopes of Planetary Nebulae and Proto-Planetary Nebulae
