Left panel: distribution of the projected distance D of stars with V ≥ 16.9 on the fiducial plotted in the top right panel. The dotted-dashed line separates stars belonging to the two different N groups. Top right: corrected C-N anticorrelation for stars with V ≥ 15.7. We tentatively discriminated between first (blue) and second (red) populations. The error bar represents the typical errors on A(C) and A(N). Middle right: CN-strong and CN-weak stars are plotted in the δA(C)-δA(N) plane. Bottom right: first- and second-generation stars are superimposed to V, (U − V) CMD of M 2. The color code is used consistently in each panel.