Fig. 1. Colour-magnitude diagrams for our targets (Cousins system). The stellar X-ray candidate members of the Coma Berenices cluster are indicated with filled circles, while three other stars also investigated in this paper are plotted with triangles. Objects are labelled as in the first column of Table 2. Error bars indicate 2 uncertainties in the photometry; errors in the y-axis are of the same size than the symbols. The cluster photometric sequence is given by previously known members represented with open circles in the figures. Overplotted to the data are the main sequence (solid line) from late-A to mid-M type field stars and the 600 Myr-isochrone (dashed line) by D'Antona & Mazzitelli (1998).