Correction of S(3839) and G4300 for stellar surface temperature and gravity for 28 member and good quality (S/N_CN≥ 25) RGB stars of NGC 3201. We fitted a second-order polynomial to each dataset, which is represented by the red dashed lines. The difference between the indices and the fitted line for a given magnitude defines δS(3839) and δG4300. Symbols arethe same as in Fig. 2. If the peculiar stars indicated by the crosses are not considered, two groups of stars with anti-correlated CN and CH can be identified: S1, indicated by squares, and S2, indicated by diamonds (see also Fig. 5 and discussions). Error bars are of the order of the point size, and are thus omitted in the plots.