Program: GS-2008A-Q-11

Phoenix spectra of C$_2$, CN and CO lines will provide oxygen isotopic abundance ratios ($^{16}$O/$^{17}$O/$^{18}$O) and CNO abundances for cool R Coronae Borealis (RCB) stars. The ratios, especially the $^{16}$O/$^{18}$O ratio, will be used to test the two leading scenarios for RCB production. These observations were prompted by Clayton et al.'s striking discovery of low $^{16}$O/$^{18}$O ($<$1) ratios for H-deficient carbon (HdC) stars. This ratio requires the formation of HdC stars by the merger of two white dwarfs as some $^{14}$N is burnt to $^{18}$O. The HdC are possible immediate relatives of the RCBs. Yet, our Phoenix observations of the cool RCB S Aps give a higher ratio ($^{16}$O/$^{18}$O=16) than for the HdCs perhaps questioning a direct evolutionary connection between HdC and RCB stars. Also, our Phoenix CNO abundance analysis of HdCs shows the $^{14}$N to $^{18}$O conversion is far from complete. In this proposal, we seek to extend our CNO elemental and isotopic abundance analyses from one RCB to several in order to explore the evolutionary connection between HdC and RCB stars and to constrain the nucleosynthesis that occurs during the cannibalism of a He white dwarf by a C-O white dwarf.