Nitrogen abundance versus projected rotational velocity. For all panels, symbols denote luminosity class and colours spectral type; see legend in the top panel. Top panel: entire sample. The newly detected binaries have a yellow circle underlaid (see Sect. 3.4). Middle panel: subset of stars for which the nitrogen abundance could be measured. Bottom panel: subset of stars for which only upper limits could be derived. The dashed lines indicate detection limits for different S/N and are discussed in Sect. 4. The black solid lines are evolutionary tracks for the O-type stage (T_eff ≥ 29 kK) of a 20 M_⊙ star (Brott et al. 2011a), with initial rotational velocities of 114, 170, 282 and 390 kms^-1. We multiply the rotational velocity of these tracks by π/ 4 to account for the average projection effect.