RS Cvn systems have been studied very extensively at radio wavelengths and a large fraction of them have been found to be strong radio sources, with 6 cm radio luminosities in the range between 1015 and 1018 erg s−1. The radio flux density is highly variable and it usually shows two different regimes: active periods, characterized by a continuous strong flaring which can last for several days, and quiescent periods, during which the flux density goes down to a few mJy. These characteristics indicate the non-thermal nature of the radio emission which is driven by magnetic activity of these systems, seen in other spectral regions. In this scenario, the radio flux arises from the interaction between the magnetic field of one or both components with mildly relativistic particles, i.e. gyrosynchrotron emission.
Due to the erratic nature of radio flares in RS CVn binaries, most VLBI observations have been performed during quiescent periods. Only the 1983 UX Ari observation (Mutel et al. 1985), carried out during an intense outburst, allowed the first, and still the unique, hybrid map of a RS CVn binary, which revealed a core-halo morphology of the radio source.
It is still not clear if the flare events are localized in very compact regions of the corona of one component or if the entire binary system is involved. VLBI measurements in both active and quiescent periods are extremely important to determine the characteristic dimensions and the structure of the radio source.