  Many early-type stars are in binary systems. A number of them shows radio
emissivity with periodic variability. This variability is associated with
non-thermal synchrotron radiation emitted by relativistic electrons. The strong
shocks necessary to accelerate the electrons up to high energies are produced
by the collision of the radiatively-driven stellar winds. A study of the
non-thermal emission is necessary in order to investigate O-star colliding wind
binaries. Here preliminary results of our modeling of the colliding winds in
Cyg OB2 No.9 are presented.
