Nitrogen surface abundance (in units of 12 + log(N/H)) as a function of projected rotational velocity. Single O stars are the blue squares and binaries the red triangles. The small black triangles (pentagons, open circles) are the Galactic B stars of (Hunter et al. 2009; Morel et al. 2008; Przybilla et al. 2010) with log g larger than 3.5 (i.e. comparable to our sample). The grey area corresponds to the range of solar values from studies of the Sun, B stars and HII regions (see Table 3 of Hunter et al. 2009).