The very rich massive post-main-sequence star population of the open   cluster Westerlund 1
  We report the discovery of a population of Wolf-Rayet stars in the young Galactic open cluster Westerlund 1. In an incomplete shallow spectroscopic survey, we find six nitrogen-rich (WN) and five carbon-rich (WC) WR stars. We also confirm the presence of a large population of yellow supergiants, some of which are candidate hypergiants. Given this population, Westerlund 1 is likely to be one of the most massive young clusters in the Local Group. 
Ignacio Negueruela (Alicante) & J. Simon Clark (UCL)