Cumulative distribution plot showing the fraction of dLHdC and RCB stars that have ¹⁶O/¹⁸O below a given value. Of the dLHdC stars, 85% (6 out of 7) have ¹⁶O/¹⁸O < 0.5 (black dashed line) while only ≈15% of RCB stars (5 out of 33) are consistent with such low ¹⁶O/¹⁸O values. Most dLHdC stars have a lower ¹⁶O/¹⁸O than most RCB stars. A166 is the only dLHdC star that has ¹⁶O/¹⁸O > 1. This star is an outlier amongst the newly discovered dLHdC stars, and is likely an RCB star in a low phase of dust production (see Sec. 4.1).