  R144 is a WN6h star in the 30 Doradus region. It is suspected to be a binary
because of its high luminosity and its strong X-ray flux, but no periodicity
could be established so far. Here, we present new Xshooter multi-epoch
spectroscopy of R144 obtained at the ESO Very Large Telescope (VLT). We detect
variability in position and/or shape of all the spectral lines. We measure
radial velocity variations with an amplitude larger than 250 km/s in NIV and NV
lines. Furthermore, the NIII and NV line Doppler shifts are anti-correlated and
the NIV lines show a double-peaked profile on six of our seven epochs. We thus
conclude that R144 is a double-lined spectroscopic binary. Possible orbital
periods range from 2 to 6 months, although a period up to one year is allowed
if the orbit is highly eccentric. We estimate the spectral types of the
components to be WN5-6h and WN6-7h, respectively. The high luminosity of the
system (log Lbol/Lsun ~ 6.8) suggests a present-day total mass content in the
range of about 200 to 300 Msun, depending on the evolutionary stage of the
components. This makes R144 the most massive binary identified so far, with a
total mass content at birth possibly as large as 400 Msun. We briefly discuss
the presence of such a massive object 60 pc away from the R136 cluster core in
the context of star formation and stellar dynamics.
