Evolution of the N/O surface abundance ratio as a function of the mass-loss rate for a 55 M_⊙ star of solar metallicity and for initial rotation rates Ω / Ω_crit from 0 to 0.4, using the models of Ekström et al. (2012). The dashed lines correspond to the adopted value of N/O, with its errors, and the lower limit for the mass-loss rate. The thicker lines emphasize the part of the tracks compatible with the measurements. For clarity, the tracks are stopped during the He burning phase (data point n^o 195 in Ekström et al. 2012).