R Coronae Borealis and dustless Hydrogen-deficient Carbon stars have
  different oxygen isotope ratios
  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 $^{18}$O. 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. We present medium
resolution (R$\approx3000$) near-infrared spectra of 20 newly discovered dLHdC
stars. We confirm that unlike RCB stars, dLHdC stars do not show strong
blueshifted ($>200$ km s$^{-1}$) He I 1.0833 $\mu$m absorption, suggesting the
absence of strong, dust-driven winds around them. We also present medium
resolution (R$\approx3000-8000$) $K$-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 conclude that most dLHdC stars
have lower $^{16}$O/$^{18}$O than most RCB stars. This confirms one of the
first 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.
