Left panel: removal of gravity and temperature dependencies from CN index using median ridge line (shown as red dashed line). Stars considered to have mid-strength or strong CN bands are depicted by filled circles, while CN-weak giants are represented by open circles. A probable non-member star (see Sect. 3) is shown as an open square. The median errorbar on the S(3839) and CH(4300) measurements is also shown in the lower-right corner of each panel. Right panel: the same as in the left panel but for the CH index. The color code is consistent with the left panel.