  We report the discovery that R139 in 30 Doradus is a massive spectroscopic
binary system. Multi-epoch optical spectroscopy of R139 was obtained as part of
the VLT-FLAMES Tarantula Survey, revealing a double-lined system. The two
components are of similar spectral types; the primary exhibits strong C III
4650 emission and is classified as an O6.5 Iafc supergiant, while the secondary
is an O6 Iaf supergiant. The radial-velocity variations indicate a highly
eccentric orbit with a period of 153.9 days. Photometry obtained with the
Faulkes Telescope South shows no evidence for significant variability within an
18 month period. The orbital solution yields lower mass limits for the
components of M1sin^3 i = 78 \pm 8 Msun and M2sin^3 i = 66 \pm 7 Msun. As R139
appears to be the most massive binary system known to contain two evolved Of
supergiants, it will provide an excellent test for atmospheric and evolutionary
models.
