Classical HRD and surface N abundances as a function of log L∕L_⊙ and T_eff for the sample stars plotted against the current Geneva (left) and the Bonn tracks (right) with rotation. Symbol coding is used to distinguish between objects of different LC; magnetic stars and fast rotators are additionally highlighted by large squares and circles, respectively. In the lower three panels, different colours denote the morphologically peculiarly objects with very weak nitrogen lines (grey), and the stars whose predicted and observed [N] values differ by less (yellow) or more (green, orange) than 1σ. In the upper panels, the solid part of the tracks corresponds to the O-type phase (T_eff ≥ 29 kK). The solar N abundance (from Asplund et al. 2005) is indicated by dotted lines.