Panel a) example of the windows from which we measured the CN and CH indices (dark gray hatched regions) together with their respective continuum windows (light gray hatched regions). Panels b) and c) show the windows adopted for the H and K calcium index and the H_β index. The non-normalized superimposed spectrum (star 41213, S/N ~ 35 in the S3839 region) was smoothed for clarity. Panel d) observed (small black dots) and synthetic (line) spectra around CH band for the star 15490. The black best fits, while the red and green lines are the syntheses computed with C abundance altered by  ± 0.10 dex from the best value. Panel e) the same as in panel d) but for the CN feature for the star 40062. The synthetic spectra show the best fit (thick black line) and the syntheses computed with N abundance altered by  ± 0.20 (thick red and green lines).