Oxygen Isotopic Ratios In Cool R Coronae Borealis Stars

We investigate the relationship between R Coronae Borealis (RCB) stars and hydrogen-deficient carbon (HdC) stars by measuring precise (16)O/(18)O ratios for five cool RCB stars. The (16)O/(18)O ratios are derived by spectrum synthesis from high-resolution (R similar to 50,000) K-band spectra. Lower limits to the (16)O/(17)O and (14)N/(15)N ratios as well as Na and S abundances (when possible) are also given. RCB stars in our sample generally display less (18)O than HdC stars-the derived (16)O/(18)O ratios range from 3 to 20. The only exception is the RCB star WX CrA, which seems to be an HdC-like star with (16)O/(18)O = 0.3. Our result of a higher (16)O/(18)O ratio for the RCB stars must be accounted for by a theory of the formation and evolution of HdC and RCB stars. We speculate that a late dredge-up of products of He burning, principally (12)C and (16)O, may convert an (18)O-rich HdC star into an (18)O-poor RCB star as the H-deficient star begins its final evolution from a cool supergiant to the top of the white dwarf cooling track.