16755

Binary population synthesis models predict that virgul30% of massive stars will have their hydrogen envelopes removed via interaction with a binary companion. The resulting ''stripped.. helium cores are relatively long-lived comma hot comma compact comma and emit extreme ultraviolet radiation. As a result comma they represent a critical piece in our understanding of the ionizing radiation from stellar populations as well as the nature of supernova and gravitational wave progenitors. However comma despite their importance and predicted ubiquity comma observations have remained elusive. This is now changing. Using UV maps of the Magellanic Clouds comma a sample of candidate systems has been identified. Optical spectroscopy reveals stars with temperatures in excess of 60 comma 000 K comma but luminosities intermediate between Wolf-Rayet and subdwarf stars. A lack of strong emission line features indicates their mass loss rates must be more than an order of magnitude lower than previously assumed based on extrapolations from higher luminosity stars. Here comma we propose to obtain HST-COS spectra of a sample of 7 of these helium stars in the Magellanic Clouds. UV observations are necessary to measure their mass loss and wind parameters comma and this sample was chosen to allow the first observational constraints of these parameters as a function of both luminosity and metallicity. Precise mass loss parameters will have a large impact on the stellar radii comma ionizing radiation comma and final surface composition of stripped helium stars. These HST-COS observations will therefore provide vital constraints for models of binary evolution comma impacting our understanding of supernova progenitors comma gravitational wave sources comma and ionizing radiation.