Stars with helium envelopes.

Models are computed for stars with helium envelopes and carbon-oxygen cores, covering a wide range of total mass and of core mass. Three types of models occur, characterized by a) non-degenerate isothermal cores, b) degenerate cores and c) carbon burning cores. There is a group of models which for the same mass and the same chemical proffle can belong either to type b) or type c) contradicting the Vogt-Russell Theorem. The models are compared with observed helium and R CrB stars indicating their interior structures and therefore their possible evolutionary history. Key words: stellar evolution - helium stars - R CrB stars