Positions of the sample stars in the spectroscopic log(T_eff)–log(ℒ/ℒ_⊙) HR diagram. The UVES and FEROS samples are shown with black and blue symbols with error bars, respectively. Regions of the parameter space covered by the GSSP, TLUSTY, and CMFGEN models are marked with red, blue, and grey shaded areas, respectively. Non-rotating mesa stellar evolution tracks computed for Ζ = 0.006 are shown with black solid lines and are taken from Burssens et al. (2020). Left panel: full parameter range. Right panel: zoom onto the high-luminosity region where the bulk of the sample is situated.