Same as Fig. 5 for the z′ − J colour. Additional objects are shown (circled symbols). They are our candidates with spectroscopic follow-up. For these objects, z′ − J is the observed colour and the absolute magnitude is derived from their spectral type. The dotted line shows the derived absolute magnitude-colour relation valid for the early-T dwarfs. Error bars are indicated when larger than the symbol. Note that only objects with measured parallax – non circled objects – are used to derive the relation.