(J − K_s, i − J) colour–colour diagram showing the positions of our subdwarf candidates identified in the SDSS/UKIDSS search (filled circles) with their respective error bars. Overplotted as filled red triangles are two known ultracool subdwarfs from Lépine & Scholz (2008) with J and K photometry from UKIDSS LAS DR5 (error bars are smaller than the symbols) and the solar-metallicity M/L dwarf sequence (line with crosses; spectral types are labelled; 0  ≡  M0, 10  ≡  L0) from West et al. (2008) and Schmidt et al. (2010b). The photometry shown in this diagram is in the MKO photometric system (Hewett et al. 2006).