Surface helium mass fraction as a function of time. The evolutionary models are shown with black lines with decreasing initial mass from left to right. The model with M_ini =  500 M_⊙ is highlighted with thicker line. The colour-coding shows the interpolated models as a function of their current mass on a logarithmic scale. The observed surface helium mass fractions by Brands et al. 2022 are indicated for the three WNh stars (R136a1 – gray, R136a2 – magenta, R136a3 – navy, see also Table 2). The shaded region shows the associated uncertainty.