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. ...