Spectroscopic HRD of the 180 Galactic O-type stars studied, with SSEM-outliers marked by star symbols. Orange stars indicate those that remain outliers even when assuming Eggenberger et al. (2021) models (subsolar baseline abundances). HD 226868, the optical counterpart of Cyg X-1, is circled in green. Non-rotating and v_ini/v_crit = 0.4 evolutionary tracks from Ekström et al. (2012) are shown for reference (see text for details). Typical uncertainties in T_eff and log ℒ are represented by a gray cross.