RS CVn stars in the ultraviolet

RS Canum Venaticorum stars are a class of binaries named after their prototype, consisting, in the most generic case, of a main-sequence star of spectral type around G-K and an evolved companion, subgiant or giant, with spectral type around K. The common features to all these stars are the presence of distortions in the light curve, not attributable to eclipses or pulsation, and their large atmospheric activity levels compared to those typically observed in the Sun. A tribute to this kind of stars is the fact that the first and the last IUE spectra taken from VILSPA, actually the very last one of the mission, correspond to Capella (α Aur), the brightest RS CVn system!. The ground-breaking work carried out by IUE on these objects has allowed a deep understanding of their upper chromosphere and transition region, through the modelling of the layers with temperatures between 10,000 and 200,000 K, where emission lines from neutral to medium ionized species originate. We have been capable to infer atmospheric inhomogeneities, using rotational modulation techniques, and detect the presence of active regions on the surface of some of these stars. In some cases, Doppler imaging techniques have also allowed a reconstruction of the distribution of bright and dark regions on the surfaces of both components. In the broad context of stellar activity, IUE has shown that RS CVn stars follow the flux-flux and the flux-rotation relations in the same qualitative way as single evolved stars do. These and other topics will be reviewed in this paper, making emphasis in the most important results.