  Massive binaries are crucial laboratories that allow us to investigate
processes occurring in quite extreme conditions, such as particle acceleration,
high-energy emission, or even dust formation. All these processes are
intimately dependent on binarity. Our understanding of the underlying physics
strongly requires preliminary multiplicity studies likely to uncover still
undetected binaries, and determine their orbital parameters. However, classical
spectroscopic approaches sometimes fail to provide a solution to this issue.
Long baseline interferometry turns out to be a promising complementary
technique to address the question of the multiplicity of massive stars. A
campaign has been initiated with the VLTI to take benefit of this technique.
