Left panels: HR diagram for BPASS models (primary stars only) with initial periods of log(P/days) = 1.4, 2.0, 2.4, 3.0, 3.2, 3.4, where P is in days. Initial masses are 9, 15, 20 and 30 M_⊙ with a mass ratio of 0.9. Blue and green indicate first and second BSG stages respectively. 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 the same models as in the left panels. The colours have the same meaning as the left panels. Black triangles represent observations of individual BSGs. The observations are taken from NGC 300 (Kudritzki et al. 2008b), other galaxies (Kudritzki et al. 2008a), M 33 (Vivian et al. 2009), M 81 (Kudritzki et al. 2012), WLM (Urbaneja et al. 2008), NGC 3109 (Hosek et al. 2014), NGC 3621 (Kudritzki et al. 2014) and NGC 4258 (Kudritzki et al. 2013). Light grey dashed lines join the first and second BSG stages from the same model.