Definition of CN and CH indices S(3839) and G4300 from Eqs. (1) and (2) using spectra of two NGC 3201 stars. Thin lines are from the CN-weak, CH-strong star N3201centre_08, thick lines are from the CN-strong, CH-weak star N3201centre_11, both S1 stars with V ≈ 13 mag. The index region is highlighted by solid blue lines and pseudo-continuum regions are identified by dashed blue lines. Spectra were locally normalised as explained in Sect. 2.2.