Sample spectra for the stellar types relevant to this work. These are (from top to bottom): a metal-poor CN-strong star, a more metal-rich CN-strong star, and two CN-normal stars with the same stellar parameters as the former stars. Metal-rich stars above [Fe/H] = −1.3 were rejected from our sample because of the risk of misclassifications. The red (blue) and gray shaded areas indicate the line and continuum bandpasses for the S(3839) and S(CH) index after Norris et al. (1981) and Martell et al. (2008), see Eqs. (1) and (4).