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 \pm 4.8 M _ { \odot } and 36.3 \pm 1.7 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 \pm 2.5 ° and absolute masses of 103.6 \pm 10.2 M _ { \odot } and 58.3 \pm 3.7 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 .