Colour–colour diagram of the observed stars (2MASS colours transformed to the Bessel & Brett (1988 system; see text). The intrinsic main sequence (drawn; shown for spectral types K0 and later) and giant branch (dashed; from type G4 at the bottom to M5 at the top) are taken from Bessel & Brett (1988). Along the main sequence we indicate the locations of unreddened stars of type M0 V to M6 V with crosses and label two of these. The direction of interstellar reddening is indicated by the dotted lines starting at types M0 and M6, where black crosses indicate values of visual extinction A_V = 1 and 2 mag.