The progeny of binary white dwarf mergers; 
extreme helium stars,  R CrB stars and subdwarf B stars

C.S. Jeffery, Armagh Observatory
H. Saio, Tohoku University

We have examined the evolution of merged low-mass double white dwarfs.
The merging process is approximated in the same way for both a helium
(He-WD) and a carbon-oxygen white dwarf (CO-WD) accretor.  After a
helium shell flash, both products become yellow giants.  Subsequent
evolution depends on the accretor composition.

The He-WD accretor contracts towards the helium main-sequence where it
will become a subdwarf B star. Before that, it briefly acquires
properties identical to those of the extreme helium star V652 Her. It
is suggested that the small fraction of {\it single} subdwarf B stars,
including those that are helium-rich, are the product of such mergers.

The CO-WD accretor evolves blueward almost horizontally in the HR
diagram.  Specific models agree very well with six different
observables of extreme helium stars.  This model provides the best
explanation for the creation of extreme helium stars and, by
association, the majority of R Coronae Borealis stars. It may also
produce a significant number of high-mass single white dwarfs.