VARIABILITY OF QUIESCENT AND FLARING RADIO-EMISSION IN RS CVN AND ALGOL-TYPE BINARY-SYSTEMS

The temporal evolution of the 5 GHz emission of RS CVn and Algol type binaries is studied. About 70 hours of VLA observations of six systems are used. It is shown that the emission is time variable both during flares and during a significant fraction of the ''quiescent'' phases, on time scales ranging from tens of minutes to some hours. Observation s of Algol during two successive passages through the quadrature point s provide evidence for a recurrent behaviour at these orbital phases. The observed time scales are qualitatively discussed with respect to s everal possible processes. The directivity of optically thin and thick gyrosynchrotron emission might account for the recurrent phase-depend ent behaviour. Variations shorter than approximately 1 h are probably related to the evolution of the radiating electrons. The occultation o f radio sources with substellar size might, however, act on similar ti me scales and must not be ignored even if definite evidence for such f eatures does not exist. The comparison between the observed short time scales of variation and the expected life times suggests that the per manent radio emission of these binary systems is due to the continuous or repetitive energization of electrons in these stellar coronae duri ng and outside large flares.