Effective temperature versus near-infrared colours of our sample of subdwarfs. The different colours stand for different metallicities: [Fe/H] = −0.5 to −0.7 dex (red), [Fe/H] = −1.0 to −1.2 dex (green), [Fe/H] = − 1.3 to −1.6 dex (blue), [Fe/H] = −1.7 dex (brown). The lines are from evolution models from Baraffe et al. (1998) at different metallicities (red: −0.5 dex, green: −1.0 dex, blue: −1.5 dex, brown: − 2.0 dex) assuming an age of 10 Gyr.