Evolutionary models for R CrB stars.

Stellar models with carbon-enriched helium mixtures have been evolved from different starting models (e.g. homogeneous models or such having a carbon-oxygen core). This kind of chemical composition is found in variable stars of type R CrB, extreme helium stars and hydrogen deficient carbon (HdC) stars. It has been investigated, for which stellar mass it is possible to evolve the models into the region of observed effective temperatures and luminosities for R CrBs. The resulting evolutionary tracks cover the whole region of observed R CrB stars and are also applicable as models for extreme helium and hydrogen deficient carbon stars.