  We present multi-epoch spectroscopic observations of the massive binary
system WR21a, which include the January 2011 periastron passage. Our spectra
reveal multiple SB2 lines and facilitate an accurate determination of the orbit
and 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 of
WR21a. Using disentangled spectra of the individual components, we derive
spectral types of O3/WN5ha and O3Vz~((f*)) for the primary and secondary,
respectively. Using the spectral type of the secondary as an indication for its
mass, we estimate an orbital inclination of $i=58.8\pm2.5^{\mathrm{o}}$ and
absolute masses of $103.6\pm10.2 \ M_{\odot}$ and $58.3\pm3.7 \ M_{\odot}$, in
agreement with the luminosity of the system. The spectral types of the WR21a
components indicate that the stars are very young (1$-$2 Myr), similar to the
age of the nearby Westerlund 2 cluster. We use evolutionary tracks to determine
the mass-luminosity relation for the total system mass. We find that for a
distance of 8 kpc and an age of 1.5 Myr, the derived absolute masses are in
good agreement with those from evolutionary predictions.
