  The nitrogen to carbon (N/C) and nitrogen to oxygen (N/O) ratios are the most
sensitive quantities to mixing in stellar interiors of intermediate and massive
stars. We further investigate the theoretical properties of these ratios as
well as put in context recent observational results obtained by the VLT-FLAMES
Survey of massive stars in the Galaxy and the Magellanic Clouds. We consider
analytical relations and numerical models of stellar evolution as well as our
own stellar atmosphere models, and we critically re-investigate observed
spectra. On the theoretical side, the N/C vs N/O plot shows little dependence
on the initial stellar masses, rotation velocities, and nature of the mixing
processes up to relative enrichment of N/O by a factor of about four, thus this
plot constitutes an ideal quality test for observational results. The
comparison between the FLAMES Survey and theoretical values shows overall
agreement, despite the observational scatter of the published results. The
existence of some mixing of CNO products is clearly confirmed, however the
accuracy of the data is not sufficient for allowing a test of the significant
differences between different models of rotating stars and the Geneva models.
We discuss reasons (for the most part due to observational bias) why part of
the observational data points should not be considered for this comparison.
When these observational data points are not considered, the scatter is
reduced. Finally, the N/C vs N/O plot potentially offers a powerful way for
discriminating blue supergiants before the red supergiant stage from those
after it. Also, red supergiants of similar low velocities may exhibit different
N enrichments, depending on their initial rotation during the main-sequence
phase.
