Top: CMD of the same sources as in Fig. 1. The horizontal and vertical lines mark the position of the tip of the red giant branch and the (J − K)₀ colour criteria used in Paper I. Bottom: colour–colour diagram of the same sources. The horizontal line marks the (J − H)₀ colour cut used in Paper I to remove contaminating foreground sources. The diagonal lines represent the (J − K)₀ colour boundaries used to select C- and the M-type sources.