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.
@article{arxiv.astro-ph/0404551,
title = {WR20a: a massive cornerstone binary system comprising two extreme early-type stars},
author = {G. Rauw and M. De Becker and Y. Naze and P. A. Crowther and E. Gosset and H. Sana and K. A. van der Hucht and J. -M. Vreux and P. M. Williams},
journal= {arXiv preprint arXiv:astro-ph/0404551},
year = {2009}
}