Nitrogen surface abundance versus projected rotational velocity for stars in the LMC sample of the FLAMES survey (Hunter et al. 2009) in the N11 (blue) and the NGC 2004 (green) fields. Stars with evidence for a binary companion from radial velocity variations are plotted as triangles. Black solid lines indicate regions that are helpful as a reference in later discussion (see also Paper II). Evolutionary tracks of 13 M_⊙ models are plotted for various rotational velocities, shifted by a factor π/4 to account for random inclination angles. The red line marks the nitrogen abundance reached at the end of the main sequence for 13 M_⊙ models adopting a mixing efficiency of f_c = 0.0228 (our calibration). Thin dotted lines show the location of the line, if different values for f_c are adopted, e.g. f_c = 0.015 (lower line) and f_c = 0.03 (upper line).