Hertzspung-Russell diagram with initial 13 M_⊙ stars accreting a fraction f_acc of their initial mass (colour coded) shortly after they finished core hydrogen burning (early Case B). The dotted parts of the tracks show the accretion phase and the dashed grey evolutionary track is for a single star. Plus symbols are added every 2 × 10⁵ yr after finishing core hydrogen burning to indicate in which region of the HR diagram stars burn helium in the core. The observationally inferred maximum luminosity of RSGs of log L/L_⊙ = 5.5 (Davies et al. 2018) is shown by a dotted black line. Explosion sites are indicated as well as the likely SN type. The grey bands show the hot and cool part of the S Doradus instability strip (Smith et al. 2004). For f_acc ≳ 0.50, the models burn helium as a BSG and for f_acc ≳ 0.75 they also explode as a BSG.