J vs. J − K_S (left panel) and J − H vs. H − K_S diagrams (right panel) for the point-like objects detected in Serpens Core (small grey dots). The circles with error bars are the T-dwarf candidates in this study; their ID number 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) and the CFHT_J0344+3206 mid-T dwarf candidate (green diamond) are plotted for comparison. The squares and triangles show the location of confirmed field L and T dwarfs (Chiu et al. 2006; Golimowski et al. 2004; Knapp et al. 2004), respectively, shifted to the distance of Serpens. Lines show the 5 Gyr and 3 My COND isochrones (dashed lines) and the 5 Gyr and 3 My DUSTY isochrones (continuous line).