sHR diagram of the stars in the sample plus 191 O-type stars from Hol18-22, color-coded by helium abundance. We adopted Y_He ≈ 0.10 as the lowest possible value. The bottom and right subpanels show Y_He against T_eff and log ℒ, respectively. The various subgroups of stars listed in Table 5 are indicated with red solid lines. Results considered as lower limits or degenerate are excluded. The bottom subpanel includes a horizontal dotted line at Y_He =0.13. The evolutionary tracks are the same as in Fig. 7 for Geneva models.