Observed luminosities for the VFTS sample as a function of effective temperature. Targets with an estimated nitrogen abundance, 7.2 ≤ϵ_N < 7.5 dex, are shown as asterisks and those with ϵ_N ≥ 7.5 dex are shown as triangles; all other targets are shown as crosses. Evolutionary tracks and isochrones (see caption to Fig. 3) are also shown, together with typical uncertainties in the effective temperatures and luminosity estimates.