Left: logarithm of the X-ray fluxes corrected for ISM absorption of O-type stars in IC 1805 as a function of the logarithm of the bolometric fluxes. The solid line yields the best-fit scaling relation (Eq. (1)) between the X-ray and bolometric fluxes considering the full sample of O-type stars, i.e. including binaries as well as single stars. Right: logarithm of the X-ray fluxes corrected for ISM absorption of O-type stars in IC 1805 as a function of . The solid lines yield the theoretical relation (Owocki et al. 2013) with m = 0.37 (Eq. (2)).