Relation between the X-ray flux (or its upper limit for the undetected sources) and the bolometric magnitude of the sdO stars observed at X-rays. The continuous red line represents the best-fit relation (f_X/f_bol = 10^−6.7) for the main-sequence early-type stars (Nazé 2009), while the area between the two blue lines (f_X/f_bol = 10^−6.2 and f_X/f_bol = 10^−7.2, respectively) corresponds to the dispersion of this relation.