  We have used (a) HST ACS imaging and STIS spectroscopy, (b) ground-based
PIONIER/VLT long-baseline interferometry, and (c) ground-based spectroscopy
from different instruments to study the orbit of the extreme multiple system HD
93 129 Aa,Ab, which is composed of (at least) two very massive stars in a
long-period orbit with e>0.92 that will pass through periastron in 2017/2018.
In several ways, the system is an eta Car precursor. Around the time of
periastron passage the two very strong winds will collide and generate an
outburst of non-thermal hard X-ray emission without precedent in an O+O binary
since astronomers have been able to observe above Earth's atmosphere. A
coordinated multiwavelength monitoring in the next two years will enable a
breakthrough understanding of the wind interactions in such extreme close
encounters. Furthermore, we have found evidence that HD 93 129 Aa may be a
binary system itself. In that case, we could witness a three-body interaction
that may yield a runaway star or a stellar collision close to or shortly after
the periastron passage. Either of those outcomes would be unprecedented, as
they are predicted to be low-frequency events in the Milky Way.

Solar and Stellar Astrophysics