CH₄on-CH₄off vs. CH₄off diagram for the point-like objects detected in Serpens Core (small grey dots). The continuous line and the line-filled area show the median locus of field stars and their dispersion, respectively. The squares and triangles show the location of confirmed field L and T dwarfs, respectively; their magnitudes have been shifted to the distance modulus of Serpens. The circles show the position of our the T-dwarf candidates; their ID number (see Tables 3, 4) is indicated and errors bars are within the circle size. The extinction vector is shown for A_V = 10 mag. The CH₄on-CH₄off vs. spectral type calibration relation by Burgess et al. (2009) is also reported on the y-axis.