Left panels: HR diagrams for primary stars in MESA models, with initial masses of 12, 15, 20 and 30 M_⊙, mass ratio of q = 0.9 and Z = 0.020. Blue, green and red indicate first, second and third BSG stages respectively. The dashed line indicates the period during which mass transfer takes place. Not all parts of the track crossing the BSG after a RSG stage are in green. This is because we define that a BSG has a hydrogen surface fraction X >  0.5. The maroon diamond indicates the evolutionary point at which the hydrogen surface abundance drops below 0.5. Right panels: M_bol vs. Log planes for same models as in left panels. The colours have the same meaning as the left panels. Observations sources are listed in caption of Fig. 2. Light grey dashed lines join the first and second BSG stages from the same model.