Constraints on wind clumping from the empirical mass-loss vs. metallicity relation for early-type stars

We present the latest results on the observational dependence of the mass-loss rate in stellar winds of O and early-B stars on the metal content of their atmospheres, and compare these with predictions. Absolute empirical rates for the mass loss of stars brighter than 10^{5.2} L_{o}, based on Hα and ultraviolet (UV) wind lines, are found to be about a factor of two higher than predictions. If this difference is attributed to inhomogeneities in the wind this would imply that luminous O and early-B stars have clumping factors in their Hα and UV line forming regime of about a factor of 3-5. The investigated stars cover a metallicity range Z from 0.2 to 1 Z_{o}. We find a hint towards smaller clumping factors for lower Z. The derived clumping factors, however, presuppose that clumping does not impact the predictions of the mass-loss rate. We discuss this assumption and explain how we intend to investigate its validity in more detail.