Distribution of evolutionary masses (left) and projected rotational velocities (right) of O- and B-type stars from Hunter et al. (2009a) in the NGC 2004 and N11 VLT-FLAMES fields. Top panels: the contributions from each VLT-FLAMES field are shown in yellow (N11) and blue (NGC 2004). Middle: apparently single stars. Bottom: radial velocity variables. The selection effects discussed in Sect. 3.3 are color-coded as per the legend in each plot. For the objects in light yellow we do not have nitrogen measurements for various reasons. The stars shown with gold panels are those with measured nitrogen abundances that are the primary focus of the current analysis. Not shown in the mass histogram are two O-type stars, with masses of 62 and 83 M_⊙.