Nitrogen-to-carbon ratio as a function of luminosity. Circles correspond to stars with M ≤ 23 M_⊙, squares indicate stars with 23 <M_∗< 33 M_⊙, triangles show stars with 33 <M_∗< 50 M_⊙ and pentagons show stars with M_∗ ≥ 50 M_⊙. Open (filled) symbols correspond to stars with vsin   i ≥ 200 km s^-1 (≤, resp.). Stellar evolution models taken from Brott et al. (2011a) for M_∗ = 20, 25, 40 and 60 M_⊙ are overplotted: dashed (solid, dotted) lines correspond to models with vsin   i ≳ 400 (300, 100) km s^-1.