N/O vs. N/C abundance ratio (by mass) measured for the sample stars and compared to the ratios predicted in Geneva models (Meynet & Maeder 2003). Solid line is for 120 M_⊙, dashed red line is for 85 M_⊙, dot-dashed green line is for 60 M_⊙, and dotted blue line is for 40 M_⊙. The black dot-dot-dot-dashed line is for a model with 60 M_⊙ and vsin   i = 500 km s^-1. Only the part of evolutionary tracks corresponding to the evolutionary phase where X(H)  ≥  0.4 are plotted.