2025PASP..137d4201M

Hydrogen-deficient Carbon (HdC) stars are a class of supergiants with anomalous chemical compositions, suggesting that they are remnants of CO-He white dwarf (WD) mergers. This class comprises two spectroscopically similar subclasses-dusty R Coronae Borealis (RCB) and dustless Hydrogen-deficient Carbon (dLHdC) stars. Both subclasses have a stark overabundance of ¹⁸O in their atmospheres, but spectroscopic differences between them remain poorly studied. We present high-resolution (R ≈ 75,000) K-band spectra of six RCB and six dLHdC stars, including four newly discovered dLHdC stars, making this the largest sample to date. We develop a semi-automated fitting routine to measure ¹⁶O/¹⁸O ratios for this sample, tripling the number of dLHdC stars with oxygen isotope ratios measured from high resolution spectra. All six dLHdC stars have ¹⁶O/¹⁸O < 1, while the RCB stars have ¹⁶O/¹⁸O > 4. Additionally, for the first time, we find a trend of decreasing ¹⁶O/¹⁸O ratios with increasing effective temperature for HdC stars, consistent with predictions of theoretical WD merger models. However, we note that current models overpredict the low ¹⁶O/¹⁸O ratios of dLHdC stars by two orders of magnitude. We also measure abundances of C, N, O, Fe, S, Si, Mg, Na, and Ca for these stars. We observe a correlation between the abundances of ¹⁴N and ¹⁸O in our sample, suggesting that a fixed fraction of the ¹⁴N is converted to ¹⁸O in these stars via α-capture. Our results affirm the emerging picture that the mass ratio/total mass of the WD binary determine whether an RCB or dLHdC is formed post-merger.