Zoom-in of the ¹²C¹⁶O and ¹²C¹⁸O absorption bands of all seven dLHdC stars (left) and six representative RCB stars (right) from our sample. All dLHdC stars except A166 show strong ¹²C¹⁸O absorption, comparable in strength to the ¹²C¹⁶O absorption. The RCB stars show a range of ¹²C¹⁸O absorption strengths. Shown are two RCB stars where the ¹²C¹⁸O absorption is strongest, two where it is of intermediate strength and two where the absorption is weak. Of the 33 RCB stars analyzed here, only five have strong dLHdC-like ¹²C¹⁸O absorption, while the remaining majority has intermediate or weak ¹²C¹⁸O absorption. We also plot examples of synthetic spectra that fit the observed spectra as red dashed lines. Note that the RCB star EROS2-CG-RCB4 shows ¹³C¹⁶O absorption bands (right panel).