R Coronae Borealis and dustless hydrogen-deficient carbon stars likely have different oxygen isotope ratios

Context. R Coronae Borealis (RCB) and dustless Hydrogen-deficient Carbon (dLHdC) stars are believed to be remnants of low mass white dwarf mergers. These supergiant stars have peculiar hydrogen-deficient carbon-rich chemistries and stark overabundances of O-18. RCB stars undergo dust formation episodes resulting in large-amplitude photometric variations that are not seen in dLHdC stars. Recently, the sample of known dLHdC stars in the Milky Way has more than quintupled with the discovery of 27 new dLHdC stars. Aims. It has been suggested that dLHdC stars have lower O-16/O-18 than RCB stars. We aim to compare the O-16/O-18 ratios for a large sample of dLHdC and RCB stars to examine this claim. Methods. We present medium resolution (R approximate to 3000) near-infrared spectra of 20 newly discovered dLHdC stars. We also present medium resolution (R approximate to 3000 - 8000) K-band spectra for 49 RCB stars. Due to the several free parameters and assumptions in our fitting strategy, we provide wide range estimates on the O-16/O-18 ratios of seven dLHdC and 33 RCB stars that show (CO)-C-12-O-16 and (CO)-C-12-O-18 absorption bands, and present the largest sample of such O-16/O-18 wide-range values for dLHdC and RCB stars to date. Results. We find that six of the seven dLHdC stars have O-16/O-18 < 0.5, while 28 of the 33 RCB stars have O-16/O-18 > 1. We also confirm that unlike RCB stars, dLHdC stars do not show strong blueshifted (>200 km s(-1)) He I 10 833 angstrom absorption, suggesting the absence of strong, dust-driven winds around them. Conclusions. Although we only can place wide range estimates on the O-16/O-18 and these are more uncertain in cool RCBs, our medium resolution spectra suggest that most dLHdC stars have lower O-16/O-18 than most RCB stars. This confirms one of the first, long-suspected spectroscopic differences between RCB and dLHdC stars. The different oxygen isotope ratios 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. Future high resolution spectroscopic observations will help confirm and more precisely quantify the difference between the oxygen isotope ratios of the two classes.