  The massive O4.5 V + O5.5 V binary VFTS 352 in the Tarantula nebula is one of
the shortest-period and most massive overcontact binaries known. Recent
theoretical studies indicate that some of these systems could ultimately lead
to the formation of gravitational waves via black hole binary mergers through
the chemically homogeneous evolution pathway. By analyzing ultraviolet-optical
phase-resolved spectroscopic data, we aim to constrain atmospheric and wind
properties that could be later used to confront theoretical predictions from
binary evolution. In particular, surface abundances are powerful diagnostics of
the evolutionary status, mass transfer and the internal mixing processes. From
a set of 32 VLT/FLAMES visual and 8 HST/COS ultraviolet spectra, we used
spectral disentangling to separate the primary and secondary components. Using
a genetic algorithm wrapped around the NLTE model atmosphere and spectral
synthesis code FASTWIND, we perform an 11-parameter optimization to derive the
atmospheric and wind parameters of both components, including the surface
abundances of He, C, N, O and Si. We find that both components are hotter than
expected compared to single-star evolutionary models indicating that additional
mixing processes may be at play. However the derived chemical abundances do not
show significant indications of mixing when adopting baseline values typical
for the system environment.
