Helium surface enrichment through mass loss. The predicted helium surface mass fraction Y_s, as a function of the estimated inverse mass-loss timescale Ṁ/M_hom for stars with T_eff > 30 kK and log(L/L_⊙) > 5, is compared with VFTS O and WNh stars labelled as in Fig. 2. The colour scale indicates the number of objects predicted by our population synthesis. Evolutionary tracks from Fig. 5 are indicated by coloured lines.