The color–magnitude diagram for the surveyed region. Overplotted are a 1 Myr isochrone (Baraffe et al. 1998) shifted to the distance of the ONC, the same isochrone reddened by A_V = 7 mag, and the 50% completeness limit for the data. Masses for a 1 Myr population of 1.0, 0.08, 0.02, and 0.01 M_⊙ are indicated on the lower left panel. The region used as control field is shown in the lower right corner (2.3–2.6 pc). The typical photometric and colour error are shown in each diagram as a function of object magnitude.