a) SofI J − H vs. H − K_s diagram for the northern field. b) SofI J − H vs. H − K_s diagram for the southern field. c) SofI K_s vs. H − K_s diagram for the northern field. d) SofI K_s vs. H − K_s diagram for the southern field. Only sources with K_s< 16 and photometric errors <0.3 mag have been selected for the colour–colour diagrams, whereas all sources with photometric errors <0.3 mag in H and K_s have been selected for the colour-magnitude diagrams. The datapoint density after smoothing is shown with contours in all panels. The grey triangles mark the sources whose colours have been taken from 2MASS. A few of these sources, associated with Pismis 24, are also indicated, although those labelled G1, G2, and G3 are the candidate background giant stars discussed in the text. The main sequence locus is drawn in all panels with a thick solid line (using the colours from Koornneef 1983, complemented with the absolute magnitudes from Allen 1976 for panels c), d)). Spectral types are labelled next to the ZAMS in panel c). The dashed lines in panels a), b) are reddening paths with crosses every A_V = 10 mag (from A_V = 0 mag), according to Rieke & Lebofsky (1985), while the thin solid lines are the reddening paths according to the extinction law derived by Straiz˘ys & Laugalys (2008). The dashed line in panels c), d) marks the completeness limit. The arrow in panel c) spans a reddening of A_V = 20 mag. Also shown as a full line parallel to the ZAMS in panel c) is the sequence of PMS stars 1 Myr old from the evolutionary tracks of Palla & Stahler (1999), for A_V = 10 mag, and masses in the 0.1–6 M_⊙ range. A distance of 1.7 kpc is assumed for ZAMS and isochrones.