Left panels: derived C and N abundances are plotted against the V magnitude for M 2 RGB stars. The dot-dashed vertical lines indicate the luminosity at which the RGB bump occurs (V ~ 15.7 mag). CN-strong and CN-weak stars, as defined in Sect. 3, are shown as filled and open symbols, respectively. Right panel: CN-strong and CN-weak stars (see Sect. 3) are plotted in the A(C) vs. A(N) plane for stars with luminosities fainter (top) and brighter (bottom) than the RGB bump. The color code is consistent with the left panel.