Nitrogen abundances derived for our LMC sample. Upper panel: as a function of O-star spectral type (“10−12” correspond to B0–B2; for T_eff, see Table 6). Lower panel: as a function of helium content, 12 + log (He/H) = 12 + log (Y_He). Red: luminosity class I − II; blue: III − V. Arrows indicate upper or lower limits. Alternative solutions for N11-031 (see Sect. 5) are indicated by a triangle and an asterisk for the cooler and hotter solution, respectively. The estimated (1-σ) error for log (Y_He) is 0.05 dex. The dotted lines indicate the LMC nitrogen (Hunter et al. 2007) and the average helium (see text) baseline abundances. Some objects were slightly shifted horizontally for clarity.