R CrB and hydrogen-deficient carbon stars

We also present medium resolution (R~3000-8000) K-band spectra for 47 RCB stars.We measure the ^16^O/^18^O ratios of 7 dLHdC and 31 RCB stars that show ^12^C^16^O and ^12^C^18^O absorption bands, and present the largest sample of values of ^16^O/^18^O for dLHdC and RCB stars to date. We find that six of the seven dLHdC stars have ^16^O/^18^O<0.5, while 26 of the 31 RCB stars have ^16^O/^18^O>1. We also confirm that unlike RCB stars, dLHdC stars do not show strong blueshifted (>200km/s) HeI 10833{AA} absorption, suggesting the absence of strong, dust-driven winds around them.

We conclude that most dLHdC stars have lower ^16^O/^18^O than most RCB stars. This confirms one of the first, longsuspected spectroscopic differences between RCB and dLHdC stars. Our results rule out the existing picture that RCB stars represent an evolved stage of dLHdC stars.

Instead, we suggest that whether the white dwarf merger remnant is a dLHdC or RCB star depends on the mass ratios, masses and compositions of the merging white dwarfs.