Evolution of the N/C ratio (in number) as a function of the gravity at the surface of 10 M_⊙ stellar models with υ_ini = 200 km s^-1. The value of the magnetic field is indicated. Continuous (dashed) lines are for models with interior differential (solid body) rotation. In the shaded areas, models with differential rotation predict surface velocities inferior to 50 km s^-1 during the MS phase. The full circle and square correspond to HD 16582 and HD 3360 (ζ Cas) respectively (see text for references).