RFG ratio versus the temperature class of HdC stars for all HdC stars whose class has been defined (Crawford et al. 2023). A–1 index has been given to EHe stars. The EHe (triangles), the dLHdC (diamonds), and the RCB (large dots) stars are plotted with a colour code that shows their GRVS magnitude group. The horizontal dotted line represents the threshold (RFG<150) used to select RCB stars that have undergone some photometric decline phases. The percentage of RCB stars that are above this threshold are indicated at the top of the diagram for each temperature class.