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CNO abundances of HdC and RCB stars: a view of the nucleosynthesis in a white dwarf merger

We present high-resolution (R~50,000) observations of near-IR transitions of CO and CN of the five known hydrogen-deficient carbon (HdC) stars and four R Coronae Borealis (RCB) stars. We perform an abundance analysis of these stars by using spectrum synthesis and state-of-the-art MARCS model atmospheres for cool hydrogen-deficient stars. Our analysis confirms reports by Clayton and colleagues that those HdC stars exhibiting CO lines in their spectrum and the cool RCB star S Aps are strongly enriched in 18O (with 16O/18O ratios ranging from 0.3 to 16). Nitrogen and carbon are in the form of 14N and 12C, respectively. Elemental abundances for CNO are obtained from CI, C2, CN, and CO lines. Difficulties in deriving the carbon abundance are discussed. Abundances of Na from NaI lines and S from SI lines are obtained. Elemental and isotopic CNO abundances suggest that HdC and RCB stars may be related objects and that they probably formed from a merger of a He white dwarf with a C-O white dwarf.