Hertzsprung-Russell Diagram (HRD) for the O-type stars in the VFTS sample, excluding binaries and poor quality fits (Sect. 3.4). Luminosities and temperatures are from Ramírez-Agudelo et al. (2017). Evolutionary sequences and isochrones are from Brott et al. (2011a) and Köhler et al. (2015). The solid tracks alter to dotted tracks for T_eff ≤ 29 kK. Grey crosses indicate LC V VFTS stars (Sabín-Sanjulián et al. 2014). The remaining stars are colour-coded according to ε_N, where upper limits are indicated in blue. Encircled stars have log g_c ≤ 3.75, where pink circles indicate that they occupy Box 2 and green circles that they are outside of that box (see Sect. 5.2 and Fig. 11).