Observational constraints on mixing of CNO-burning products in massive stars. Mass ratios N/C over N/O are displayed. Black dots: present data; black circles: 9 additional objects from NS11. Data from previous non-LTE analyses are given as grey symbols (green online) – plus signs: Kilian (1992); triangles: Gies & Lambert (1992); diamonds: Cunha & Lambert (1994), Daflon et al. (1999, 2001a,b); squares: Morel et al. (2008); crosses: Hunter et al. (2009). The predicted nuclear paths assuming initial CAS abundances (red star) and solar abundances ( ⊙ ) of Asplund et al. (2009) are indicated by the long-dashed and dotted lines, respectively. Statistical uncertainties of 0.2 dex in each element – which are typical for previous literature data – result in an error bars more than twice as large than those from the present data.