HRD colour-coded with the surface helium mass fraction in our models (using the Taylor-Spruit scheme without MLT++ with an initial rotational velocity of 300 km s⁻¹) and in observations of the WNh stars R136a1 (star), R136a2 (circle), and R136a3 (square). The initial mass of the evolutionary tracks is indicated in solar units. We apply smoothing to avoid numerical noise. Uncertainty ranges (lower and upper limits) in the measured helium abundances from three authors are indicated in the upper part (see also Table 2). For the nominal values, we adopt the Brands et al. (2022) results and show the lower and upper limits as the left and right side of the filled markers. The shaded regions show the formal uncertainty in logL and T_eff from Brands et al. (2022).