Close binary systems of the RS CVn and Algol type are characterized by a highly variable radio emission, from typical flux densities of a few mJy during low-activity (quiescent) periods, up to 1 Jy during strong flares. The origin of this radio emission is attributed to the gyrosynchrotron mechanism.

Several observations of these stars have shown an evolution of the radio spectrum during both the rising and decay phases of flares. Moreover, VLBI observations at different epochs show a possible evolution also of the source structure, from a core-halo during flares to an extended halo in quiescent phases. In order to interpret these observations, I have developed a model that takes into account the time evolution of an ensemble of relativistic electrons in a dipolar magnetic field. This model reproduces very well the observed characteristics of the radio emission in both phases.

Recent observations extended over several consecutive orbital periods have shown that the emitted radiation appears to be correlated with the orbital phase. It is shown that a model taking into account the combined effect of rotation and of the time evolution of the particles is able to reproduce the light curves observed on some systems during periods of stronger activity.

An important characteristic of the radio emission from these systems is the presence of circular polarization, which changes its sign between 1.4 and 5 GHz, independently of the spectral shape. This inversion is in contrast with the predictions of any gyrosysnchrotron model. The observations, at 1.4 GHz, of a rapidly varying component, which is highly polarized in a sense opposite to the high frequency emission, suggests an alternative explanation for this inversion in terms of a coherent emission superimposed on the gyrosynchrotron component. It is also shown that there is a discrepancy between the predictions of gyrosynchrotron models and the behaviour of the polarization at higher frequencies.