Density map in the (H, H − K_S) CMD of all point-like sources in the R10 catalog with measured HK_S photometry (color-filled areas), together with the 2 Myr isochrone computed by Scandariato et al. (in prep.) (solid green line, the diamond symbols correspond to 1 M_⊙ and 0.1 M_⊙ from top to bottom respectively). Two main populations are apparent: the ONC clustered at H  = 12 and the bona-fide background stars at H  ~  17. The latter population is to be compared with the synthetic galactic population provided by Robin et al. (2003) (solid contours) after accounting for the extinction provided by the OMC-1. The case of a constant A_V = 6 across the cloud is shown in dashed contours, the highest ones roughly overlapping with the observed background population. The dotted line represents our safe selection criterion: any source fainter than H  = 15 or redder than H − K_S  = 1.3 is considered a field star, provided that its colors are not compatible with the reddened Meyer’s locus (Sect. 4).