HR diagram with the isochrones of Siess et al. (2000). Filled red circles show spatially unresolved components, either close binary components of visual triple stars or spectroscopic binaries. Luminosities of these targets probably appear to be higher than single stars at the same T_eff. Open blue squares show targets with strong near-IR excess (E(K_s−L′) > 0.5 or H−K_s > 0.6) and filled blue squares all other components of the Chamaeleon I sample. In the lower left corner we present the average random error and the luminosity uncertainty introduced by 0.2 mag variability in J -band (estimated from measurements, see Sect. 2.2.2).