X-ray versus bolometric luminosity (a small number of objects, not relevant for this study, falls outside the plotted L_bol range); dotted lines represent constant L_x/L_bol ratios 10^-3, 10^-4 and 10^-5; the dashed line is the linear regression fit derived by Preibisch et al. (2005a) for the ONC corresponding to log (L_x/L_bol) =  −3.7; the solid lines are linear regression fits to the IC 348 members in the luminosity range log (L/L_⊙) =  −1.5... + 0.5, which corresponds roughly to the upper mass boundary of the Baraffe et al. (1998) models: black, red and green are for the whole sample, only Class II and only Class III sources, respectively.