Nitrogen abundance versus projected rotational velocity for the full sample. Symbols indicate luminosity class, colours indicate spectral subtype. Tracks (Brott et al. 2011a) drawn in solid black lines are for a 20 M_⊙ star, evolved until the effective temperature is below 29 kK and they become B-type stars. Initial rotational velocities for the tracks are 114, 170, 282 and 390 kms^-1 and have been multiplied by π/ 4 to account for the average projection effect. The population synthesis calculation projected in the background is for the O star phase only and assumes a flat distribution of rotational velocities. Out of 740 × 10³ simulated stars, 310 × 10³ pass the selection criterion (T_eff ≥ 29 kK). These are normalised to 72 stars, matching the number of observed stars in the plot.