  Some OB stars show variable non-thermal radio emission. The non-thermal
emission is due to synchrotron radiation that is emitted by electrons
accelerated to high energies. The electron acceleration occurs at strong shocks
created by the collision of radiatively-driven stellar winds in binary systems.
Here we present results of our modelling of two colliding wind systems: Cyg OB2
No. 8A and Cyg OB2 No. 9.
