log(f_X/f_bol) (top row) and log(L_X/L_bol) (bottom row) relation for O stars of different luminosity classes (V, III, and I from left to right panels). Known spectroscopic binaries are highlighted with a ring around the solid circle, and spectral types are color coded as in Fig. 5. The linear fits log(f_X) = a + b × log(f_bol) and the mean log(f_X/f_bol) values for each case are shown as solid and dotted lines, respectively (see text for details). For O supergiants, the correlations are weak, while they are significant for O dwarf and giant stars (see Table 3).