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 ¹⁶O/¹⁸O ratios for five cool RCB stars. The ¹⁶O/¹⁸O ratios are derived by spectrum synthesis from high-resolution (R ~ 50, 000) K-band spectra. Lower limits to the ¹⁶O/¹⁷O and ¹⁴N/¹⁵N ratios as well as Na and S abundances (when possible) are also given. RCB stars in our sample generally display less ¹⁸O than HdC stars—the derived ¹⁶O/¹⁸O ratios range from 3 to 20. The only exception is the RCB star WX CrA, which seems to be an HdC-like star with ¹⁶O/¹⁸O = 0.3. Our result of a higher ¹⁶O/¹⁸O 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 ¹²C and ¹⁶O, may convert an ¹⁸O-rich HdC star into an ¹⁸O-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.