Distributions of log (N/C) versus log(N/O) for the ON stars (large symbols) and for a comparison set of normal O stars with spectral types O8.5−O9.7 (small symbols) taken from Martins et al. (2015). The upper left panel shows the full sample. The upper right (lower left, lower right) shows the dwarf–subgiant (giant, supergiant) sample. Solid lines in the upper left panel show the relations expected for partial CN or complete CNO burning in equilibrium.