Pre-SN locations of the Cases A–C accretors in the HR diagram. The dotted line at log L/L_⊙ = 5.5 (Davies et al. 2018) is the maximum luminosity of RSGs and the grey shaded regions indicate the hot and cool parts of the S Doradus instability strip. The dashed line is the Humphreys–Davidson limit (Humphreys & Davidson 1979). The dark-grey boxes indicate the locations of the RSG N6946-BH that likely collapsed into a BH without a bright SN (Gerke et al. 2015; Adams et al. 2017a,b; Basinger et al. 2021), and the BSG progenitor of SN 1987A (Woosley 1988). The grey evolutionary tracks are single-star models with the masses given by the labels.