We report on the first results from the e-MERLIN Cyg OB2 Radio Survey (COBRaS), which is designed to exploit e-MERLIN's enhanced capabilities to conduct deep-field mapping of the tremendously rich Cyg OB2 association in our Galaxy. The project aims to deliver the most detailed radio census of the most massive OB association in the northern hemisphere.

There is considerable evidence for structure (clumping) in the radiatively driven stellar winds of hot stars. The existence of clumping has important consequences for mass-loss rate determinations. Mass-loss rates that are not corrected for clumping provide incorrect inputs for stellar and Galactic evolution models. Radio observations are ideally suited to study the effect of clumping in the outer regions of the wind. We present the first 20 cm (L-band)
continuum detections of OB stars in Cyg OB2. These data substantially increase the observational detections of the outer wind of massive stars. In combination with other observations at different wavelengths COBRaS will greatly advance our knowledge of clumping as a function of radial distance around massive stars. The observations allow us to quantify the amount of clumping and search for possible relations with stellar and/or wind parameters.