Carbon mass fraction as a function of nitrogen mass fraction for the sample stars. Red triangles, blue circles, and grey squares refer to Galactic, LMC, and SMC stars, respectively. One Galactic star is out of the frame and is seen in the insert where we show the same data points on a wider scale together with 40 M_⊙ (solid line) and 60 M_⊙ (dotted line) Geneva models at solar (pink), LMC (cyan), and SMC (grey) metallicity. The three SMC stars analysed by Martins et al. (2009) are included.