X-ray to optical flux plotted over the B − V colour for all objects of the Com field (Table 8). The straight line marks the upper boundary of the region populated by stars (Stocke et al. 1991), and the areas populated by various spectral types are taken from Beuermann et al. (1999). Thus, triangles denote secure star identifications. Accretion-powered systems (CVs, AGN) can reach high f_X/f_opt values. Since most of those systems have blue optical colours, they populate the upper left corner. The high number of objects reflects the fact that the Com field is at high galactic latitude where the fraction of AGN is high. No extinction correction has been applied, since the correction factor is smaller than a factor of two (Fig. 3). This and the error in B − V are visualized by the cross in the lower right corner.