Helium surface enrichment through rotation. The predicted helium surface mass fraction Y_s, as a function of rotational velocity for O stars with T_eff > 29 kK and 5.0 < log(L/L_⊙) < 5.9, is compared with observed Y_s and v sin i of He-enriched VFTS O stars (with Y_s > 0.35) labelled as in Fig. 2; stars with log(L/L_⊙) < 5.0 are plotted in grey. The colour scale indicates the number of objects predicted by our population synthesis models. Evolutionary tracks for an initial mass of 30 M_⊙ are indicated by coloured lines. Step functions indicate the initial (blue) and present (orange) total number distributions in arbitrary units, as well as the present distribution of He-enriched stars with Y_s > 0.35 (red).