Top: HR diagram of models with v_ini = 500 km s^-1 (chemically-homogeneous evolution) and masses between 9−294 M_⊙. The thick green line marks the ZAMS. The colouring marks the surface helium mass fraction as indicated by the legend. For Y_S< 0.55, OB-type mass-loss is applied; for Y_S between 0.55 and 0.7, an interpolation between OB- and WR-type mass-loss is applied; and for Y_S> 0.7, WR-type mass-loss is applied (cf. Sect. 2.3). Bottom: HR diagram of the same collection of models as above. The colouring marks the wind optical depth τ according to Eq. (1).