Colour-magnitude diagram K_s versus H - K_s of the stars in the upper panel of Fig. 1. The colour scale shows stellar densities using a power stretch scale. The red line indicates the colour cut applied to remove foreground stars (see Sect. 2.1). The shaded blue parallelogram indicates the selection box for the proper motion analysis described in Sect. 3.3, whereas the dashed black lines indicate the colour bins for the stellar density analysis in Sect. 4. The black arrow shows the direction of the reddening vector.