Left panel: K_S − [3.6]  vs.  [3.6]−[4.5]  color–color diagram. The circles with error bars are our the T-dwarf candidates; their ID number (see Tables 3, 4) is indicated and the arrows indicate their position after correction for interstellar reddening. The Serpens Core class II (black dots) and class III (asterisks) YSO population, the S Ori 70 mid-T dwarf (sky-blue diamond), the CFHT_J0344+3206 mid-T dwarf candidate (green diamond) and the field dwarf sequence from Patten et al. (2006) (line-filled area) are plotted for comparison. Righ panel: i − J vs. J − [3.6]  color–color diagram. Symbols are as in the left panel. The dot-dashed ellipse marks the area occupied by QSOs in the SWIRE catalog (Hatziminaoglou et al. 2008).