Nitrogen-to-carbon ratio (upper panel) and helium abundance (lower panel) versus nitrogen-to-oxygen ratio, normalised to cosmic abundance standard values (Nieva & Przybilla 2012). Objects from the present work (open symbols), B-type main-sequence stars (Nieva & Przybilla 2012, black triangles), and BA-type supergiants (Przybilla et al. 2010, black dots) are compared to predictions from stellar evolution models. Solid lines: 15 M_⊙, Ω_rot = 0.95 Ω_crit model by Georgy et al. (2013); dashed lines: 15 M_⊙, Ω_rot = 0.568 Ω_crit model by Ekström et al. (2012); dotted lines: 25 M_⊙, Ω_rot = 0.568 Ω_crit model by Ekström et al. (2012). For all tracks, a metallicity of Z =0.014 was assumed. Abundances in the models were normalised with respect to their initial model values.