Long-term chromospheric activity of non-eclipsing RS CVn-type stars

Context. The IUE database provides a large number of UV high and low-resolution spectra of RS CVn-type stars from 1978 to 1996. In particular, many of these stars were monitored continuously during several seasons by IUE. Aims. Our main purpose is to study the short and long-term chromospheric activity of the RS CVn systems most observed by IUE: HD 22468 (V711 Tau, HR 1099, K1IV+G5V), HD 21242 (UX Ari, K0IV+G5V) and HD 224085 (II Peg, K2IV). Methods. We first obtain the Mount Wilson index S from the IUE high and lowresolution spectra. Secondly, we analyse with the Lomb-Scargle periodogram the mean annual index 〈S〉 and the amplitude of the rotational modulation of the index S. Results. For HD 22468 (V711 Tau, HR 1099), we found a possible chromospheric cycle with a period of ∼18 years and a shorter cycle with a period of ∼3 years, which could be associated to a chromospheric “flip-flop” cycle. The data of HD 224085 (II Peg) also suggest a chromospheric cycle of ∼21 years and a flip-flop cycle of ∼9 years. Finally, we obtained a possible chromospheric cycle of ∼7 years for HD 21242 (UX Ari).