The very massive binary NGC3603-A1
  Using VLT/SINFONI, we have obtained repeated AO-assisted, NIR spectroscopy of the three central WN6ha stars in the core of the very young (~1 Myr), massive and dense Galactic cluster NGC3603. One of these stars, NGC3603-A1, is a known 3.77-day, double-eclipsing binary, while another one, NGC3603-C, is one of the brightest X-ray sources among all known Galactic WR stars, which usually is a strong indication for binarity. Our study reveals that star C is indeed an 8.9-day binary, although only the WN6ha component is visible in our spectra; therefore we temporarily classify star C as an SB1 system. A1, on the other hand, is found to consist of two emission-line stars of similar, but not necessarily of identical spectral type, which can be followed over most the orbit. Using radial velocities for both components and the previously known inclination angle of the system, we are able to derive absolute masses for both stars in A1. We find M_1 = (116 \pm 31) Mo for the primary and M_2 = (89 \pm 16) Mo for the secondary component of A1. While uncertainties are large, A1 is intrinsically half a magnitude brighter than WR20a, the current record holder with 83 and 82 Mo, respectively; therefore, it is likely that the primary in A1 is indeed the most massive star weighed so far. 
O. Schnurr, A. F. J. Moffat, N. St-Louis, J. Casoli, and A.-N. Chen\'e