Zoom-in of the spectra of the RCB star ASAS-RCB-7 (solid black line). We plot the best fit-model synthetic spectrum to this star (gray, dashed line), and show the contribution of ¹²C¹⁴N (solid red), ¹²C¹⁶O blue and ¹²C¹⁸O green. Unlike WXCrA (Fig. 5), this spectrum does not show very strong ¹²C¹⁸O bandheads. As our oxygen ratio measurements are based on these bandheads, we estimate ¹⁶O/¹⁸O≥500 for this source. It is possible that the low spectroscopic resolution and S/N of this spectrum cannot resolve the blended weak bandhead. Higher resolution spectra are required to measure the ratio accurately. The other targets with ¹⁶O/¹⁸O≥500 could also be affected by a similar issue.