WR20a: a massive cornerstone binary system comprising two extreme   early-type stars
  We analyse spectroscopic observations of WR20a revealing that this star is a massive early-type binary system with a most probable orbital period of \sim 3.675 days. Our spectra indicate that both components are most likely of WN6ha or O3If^*/WN6ha spectral type. The orbital solution for a period of 3.675 days yields extremely large minimum masses of 70.7 \pm 4.0 and 68.8 \pm 3.8 M_{\odot} for the two stars. These properties make WR20a a cornerstone system for the study of massive star evolution. 
G. Rauw, M. De Becker, Y. Naze, P.A. Crowther, E. Gosset, H. Sana,   K.A. van der Hucht, J.-M. Vreux and P.M. Williams