Difference between the observed [N/O] ratio and that predicted by BONNSAI, normalised by the error derived for our objects, as a function of a similar difference for the abundance y. Stars with lower/upper limits in [N/O] are not included, explaining why there are fewer stars than indicated in Tables B.1–B.3. Filled circles represent the stars common to the three panels. The three dash-dotted circles delimitate the areas in which the differences are within 1, 2, and 3σ. Colours represent the multiplicity status of our targets. The input parameters in BONNSAI are different for the three panels – left panel: full parameter set (T_eff, log g_C, vsin i, He and CNO abundances); middle panel: T_eff, log g_C, vsin i, and y; right panel: T_eff, log g_C, and vsin i.