Evolutionary tracks of RSG remnants with a BH and NS companion in the Hertzsprung–Russell diagram (panel a) and in the plane of central temperature versus central density (panel b). The solid blue and dashed orange lines in panel a indicate the evolutionary tracks of RSG remnants undergoing MT to a BH and NS companion, respectively. The black cross represents the RSG remnant at the end of the CE phase, namely the start of evolutionary tracks. The onset and termination of MT are marked with symbols of stars and triangles, respectively. Constant radii are shown by grey lines, among which the dash-dotted lines are the Roche lobe radii given the initial configuration of the post-CE binaries. In panel b, different colours mark the evolutionary points at which the corresponding abundance depletes in the centre of stars. The grey zone indicates the centre of the core is in an electron degenerate state.