z′ − J versus i′ − z′ diagram of the brown dwarf candidates in our sample with complete J-band follow-up. Open circles: dwarfs earlier than M 8. Filled circles: M 8 to L dwarfs. Open triangles: early T dwarfs. Filled triangles: late T dwarfs. The dotted line shows the colour–colour relation derived in Sect. 3.1. The solid lines show the different spectral type regions defined in Table 3. Note that to build the T dwarfs luminosity-function we did not use this colour-based classification but used the spectral type obtained with spectroscopic follow-up whenever available.