The R Coronae Borealis (RCB) stars are rare hydrogen--deficient, carbon--rich
supergiants. They undergo extreme, irregular declines in brightness of many
magnitudes due to the formation of thick clouds of carbon dust. It is thought
that RCB stars result from the mergers of CO/He white dwarf (WD) binaries. We
constructed post--merger spherically asymmetric models computed with the MESA
code, and then followed the evolution into the region of the HR diagram where
the RCB stars are located. We also investigated nucleosynthesis in the
dynamically accreting material of CO/He WD mergers which may provide a suitable
environment for significant production of 18O and the very low 16O/18O values
observed. We have also discovered that the N abundance depends sensitively on
the peak temperature in the He--burning shell. Our MESA modeling consists of
engineering the star by adding He--WD material to an initial CO--WD model, and
then following the post--merger evolution using a nuclear--reaction network to
match the observed RCB abundances as it expands and cools to become an RCB
star. These new models are more physical because they include rotation, mixing,
mass-loss, and nucleosynthesis within MESA. We follow the later evolution
beyond the RCB phase to determine the stars' likely lifetimes. The relative
numbers of known RCB and Extreme Helium (EHe) stars correspond well to the
lifetimes predicted from the MESA models. In addition, most of computed
abundances agree very well with the observed range of abundances for the RCB
class.Comment: 14 pages, 7 figures, MNRAS in pres
