Mass transfer in a binary system - The evolution of the mass-giving helium star

The evolution of helium stars with mass loss due to Roche lobe overflow is investigated. This kind of mass transfer, called case BB, can start before or after the ignition of carbon, depending on the core mass of the helium star. The results show that stars with core masses below the Chandrasekhar limiting mass for white dwarfs are able to transfer the whole envelope to the main-sequence companion, and become white dwarfs, whereas for stars with core masses above this limit a supernova explosion is unavoidable. Case BB mass transfer should also produce an observable helium overabundance in the envelopes of the primaries in some X-ray binary systems.