2007ApJ...662.1220C

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 M_☉/yr producing temperatures at the base of the accreted envelope of 1.2-1.9x10⁸ 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/^ 13^C 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.