  We have found that at least seven hydrogen-deficient carbon (HdC) and R
Coronae Borealis (RCB) stars, have 16O/18O ratios close to and in some cases
less than unity, values that are orders of magnitude lower than measured in
other stars (the Solar value is 500). Greatly enhanced 18O is evident in every
HdC and RCB we have measured that is cool enough to have detectable CO bands.
The three HdC stars measured have 16O/18O < 1, lower values than any of the RCB
stars. These discoveries are important clues in determining the evolutionary
pathways of HdC and RCB stars, for which two models have been proposed: the
double degenerate (white dwarf (WD) merger), and the final helium-shell flash
(FF). No overproduction of 18O is expected in the FF scenario. We have
quantitatively explored the idea that HdC and RCB stars originate in the
mergers of CO- and He-WDs. The merger process is estimated to take only a few
days, with accretion rates of 150 Msun/ yr producing temperatures at the base
of the accreted envelope of 1.2 - 1.9 x 10^8 K. Analysis of a simplified
one-zone calculation shows that nucleosynthesis in the dynamically accreting
material may provide a suitable environment for a significant production of
18O, leading to very low values of 16O/18O, similar to those observed. We also
find qualitative agreement with observed values of 12C/13C and with the CNO
elemental ratios. H-admixture during the accretion process from the small
H-rich C/O WD envelope may play an important role in producing the observed
abundances. Overall our analysis shows that WD mergers may very well be the
progenitors of O18-rich RCB and HdC stars, and that more detailed simulations
and modeling are justified.
