I − (R − I) (top left), I − (I − J) (top right), I − (I − K_s) (bottom left), and J − (J − K_s) (bottom right) colour − magnitude diagrams of optically selected candidates (points) from the TLS and 2MASS/UKIDSS surveys. In all CMDs, we show 500 Myr NextGen isochrones shifted to the distance of the Coma open cluster as solid lines; the same age DUSTY isochrones (lower solid line) are shown in the I − (I − J) and I − (I − K_s) CMDs. The vertical line labelled with stellar mass (M_⊙) is the mass scale according to the NextGen (Baraffe et al. 1998) models. The DUSTY isochrones in I − (I − J) and I − (I − K_s) are also marked with stellar mass. The two dashed lines display the uncertainty in the RI-photometry (PSF photometric errors only), which grows with magnitude. The upper horizontal line shows the stellar/substellar boundary and the lower line indicates the lithium burning limit. Field objects (points) from two CCD frames are shown. All objects around the I − (R − I) isochrone have been included in the list of candidates and thus the non-candidates are represented here for demonstration only. Five brown dwarf candidates are located close to or in the substellar domain.