Colour-magnitude diagram K_s versus H − K_s. The grey dots represent all the stars in the KMOS catalogue in the analysed region after excluding the foreground population. Stars with proper motions below 1 mas/yr and stars with orbits are overplotted with different colours and symbols, as indicated in the legend. The black dashed line corresponds to the cut applied in Sects. 3, 4.2 and 5 to analyse the effect of missing bright stars at K_s ~ 10 mag when applying the proper motion quality cut.