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We present multi-epoch spectroscopic observations of the massive binarysystem WR21a, which include the January 2011 periastron passage. Our spectrareveal multiple SB2 lines and facilitate an accurate determination of the orbitand the spectral types of the components. We obtain minimum masses of$64.4\pm4.8 \ M_{\odot}$ and $36.3\pm1.7 \ M_{\odot}$ for the two components ofWR21a. Using disentangled spectra of the individual components, we derivespectral types of O3/WN5ha and O3Vz~((f*)) for the primary and secondary,respectively. Using the spectral type of the secondary as an indication for itsmass, we estimate an orbital inclination of $i=58.8\pm2.5^{\mathrm{o}}$ andabsolute masses of $103.6\pm10.2 \ M_{\odot}$ and $58.3\pm3.7 \ M_{\odot}$, inagreement with the luminosity of the system. The spectral types of the WR21acomponents indicate that the stars are very young (1$-$2 Myr), similar to theage of the nearby Westerlund 2 cluster. We use evolutionary tracks to determinethe mass-luminosity relation for the total system mass. We find that for adistance of 8 kpc and an age of 1.5 Myr, the derived absolute masses are ingood agreement with those from evolutionary predictions.

The mass of the very massive binary WR21a