  The multiplicity of massive stars is known to be significantly high. Even
though the majority of massive stars are located in binary systems, the census
of binaries is biased toward shorter periods as longer period systems are more
difficult to identify. Alternatively, the search for binary systems with longer
periods may proceed differently. As massive binary systems are typically
colliding-wind systems, hints for processes occurring in the colliding-wind
region could be used as a valuable proxy to identify likely binary systems, and
then organize dedicated spectroscopic or interferometric campaigns on a short
list of pre-selected targets. In this context, any hint for synchrotron radio
emission is seen as a promising indicator of long period binaries, as short
period systems undergo severe free-free absorption of the synchrotron emission
by the stellar wind material. Usual techniques to identify synchrotron radio
emitters constitute thus valid tools to explore that poorly investigated part
of the massive binary parameter space. In addition, the identification of a
synchrotron emission component in a short period binary can be used as an
indicator of the presence of a third companion on a still unrevealed wider
orbit in a triple system.
