Core-mass–luminosity relation (panel a) and the relationship between the effective temperature and the average hydrogen mass fraction in the envelope (panel b) of single stars and Cases A–C accretors at core collapse. In the core-mass–luminosity relation, only CSG Cases B and C accretors are shown as the BSG accretors with their thick hydrogen-burning shell follow a different mass-luminosity relation (see main text and Fig. 11). The dashed black curve in panel a is the fit of the core-mass–luminosity relation from Temaj et al. (2024).