Left panel: CMD of the C1+C2−1σ+C3 selected data sample. The main ridge line is shown as a gray line. Middle panel: CMD rotated along the slope of the main ridge line. Right panel: histograms of the distances in color from the main ridge line. The black solid line corresponds to the data sample after applying all selection criteria (C1, C2 1-σ and C3). The vertical line at Δ(r − i)=0 shows the position of the mean ridge line which serves as a reference line. It also represent the location of the single stars belonging to the main population of the ONC. The two vertical lines Δ(r − i) > 0 indicate the expected positions of the equal mass binary and triple systems, respectively. The red dashed histogram shows the unresolved binary stars from Tobin et al. (2009). The solid-line red histogram shows the distribution of the unresolved binaries that passed the selection criteria (C1, C2 1-σ and C3).