The colour − magnitude diagrams for the four studied clusters. The Schmidt-Kaler (1982) sequence is shown in the (J − K_s) vs. K_s diagram. Red circles mark the stars for which spectra have been observed, and the number attached to them refers to the name of the star listed in Table 2. The red dashed lines represent 50% completeness limit. The green line is the main-sequence model (the exact colour changes with the age used), whereas the others (light blue, dark blue, orange and read) are PMS models. See text for more details.