Subsample of 246 Galactic O- and B-type stars investigated by Simón-Díaz et al. (2017), color-coded by the LC taken from Simbad. Left panel: location of the stars in a spectroscopic Hertzsprung-Russel diagram (see Sect. 3.1.1). The rough boundary between the O- and B-type star domains is indicated with the dotted diagonal black line. The evolutionary tracks taken from the MESA Isochrones & Stellar Tracks online tool for solar metallicity, and no initial rotation are included for reference purposes. Right panel: log(ℒ/ℒ_⊙) against FW3414(Hβ) for the same stars. The vertical dashed red line and dashed horizontal black lines at log(ℒ/ℒ_⊙) = 3.5 dex in both panels are included for reference (see Sect. 4.1).