Matches between the foreground contamination and well-known catalogs of the ONC. a) In green, the optical spectroscopic survey of Tobin et al. (2009), matches with the foreground population are marked in blue. b) In yellow, the well-studied sample of Hillenbrand (1997), matches with the foreground population are marked in blue. c) In red, the sample of X-ray sources from the COUP project (Feigelson et al. 2002), matches with the foreground population are marked in blue. As in Fig. 5, the unshaded area represents the region of the cloud where A_V ≥ 5 mag, on which the selection of foreground sources was performed.