Left: Binary evolution tracks of a 20 M_⊙ primary star at Z = 0.008 and right: at Z = 0.002 undergoing thermal mass transfer during the expansion phase. The two rows correspond to different value of semi-convection parameter used (α_sc = 0.01 and 100). The colours of the binary tracks have the same meaning as in Fig. 1. The green diamonds mark end of core-He burning. The two grey dashed lines are the H- and He-ZAMS with different initial masses marked as grey squares. Black circles (A–D) are marked where the models regain thermal balance after the MT phase. The two blue dotted curves show loci of structure models with increasing H envelope mass on top of a 7 M_⊙ pure-He core, corresponding to the different mass fraction of H added: X = 0.1, 0.2. These structure models are over-luminous compared to their He-ZAMS counterparts of the same mass, and illustrate the inflated nature of models with a low-mass, H-poor envelope above a He-burning core.