Model fits to the CO absorption bandheads of the dLHdC star A166 and RCB stars ASAS-RCB-17, WXCrA and ASAS-RCB-7. The spectra (with errorbars) are plotted in black, and the best-fit models for a wide allowed range of model parameters (Table 2) are plotted as solid colored lines. The measured ¹⁶O/¹⁸O for these stars are > 20, 5 − 50, 0.05 − 1 and > 500 respectively. The dotted and dashed lines show models with oxygen isotope ratios outside these derived ranges (¹⁶O/¹⁸O = 10 for A166, 1 and 500 for ASAS-RCB-17, and 5 for WXCrA). It is challenging to distinguish between models with different N abundances given the medium resolution and S/N of the spectra. We focus on the regions around the ¹²C¹⁸O and ¹²C¹⁶O bandheads (dashed green and gray lines) and report the range of values consistent with the observed spectra.