Program: GS-2006A-C-13

The hydrogen-deficient carbon star HD 137613 has been reported by Clayton et al.~(2005) to have the extraordinary isotopic ratio $^{16}$O/$^{18}$O $\sim$ 0.8. The solar system ratio is $\sim$500 and carbon stars typically have yet larger ratios. The explanation proposed for the $^{16}$O/$^{18}$O value observed in HD 137613 is that the HdC stars are recently merged white dwarf binaries. The observed supergiant HdC star is a transient post-merger phase. If so, we have the rare opportunity to see helium-burning shell products in a low gravity stellar environment. To test this model we propose to measure the CNO elemental abundances for HD 137613 as well as the other four hydrogen-deficient carbon stars. Comparison of the measure abundances with models will lead to considerable insight into stellar mergers and nucleosynthesis.