  Context. A significant number of the Wolf-Rayet stars seem to be binary or
multiple systems, but the nature of many of them is still unknown. Dedicated
monitoring of WR stars favours the discovery of new systems.
  Aims. We explore the possibility that WR62a is a binary system.
  Methods. We analysed the spectra of WR62a, obtained between 2002 and 2010, to
look for radial-velocity and spectral variations that would suggest there is a
binary component. We searched for periodicities in the measured radial
velocities and determined orbital solutions. A period search was also performed
on the "All-Sky Automated Survey" photometry.
  Results. We find that WR62a is a double-lined spectroscopic binary with a WN5
primary star and an O 5.5-6 type secondary component in orbit with a period of
9.1447 d. The minimum masses range between 21 and 23 Mo for the WN star and
between 39 and 42 Mo for the O-type star, thus indicating that the WN star is
less massive than the O-type component. We detect a phase shift in the
radial-velocity curve of the He II 4686 emission line relative to the other
emission line curves. The equivalent width of this emission line shows a
minimum value when the WN star passes in front of the system. The analysis of
the ASAS photometry confirms the spectroscopic periodicity, presenting a
minimum at the same phase.
