Chromospherically active stars: Chemical composition of photospheres in 20 RS CVn stars

Context. Various element transport processes modify the photospheric chemical composition of low-mass stars during their evolution. The most prominent one is the first dredge-up that occurs at the beginning of the red giant branch. Then, various extra-mixing processes, such as those caused by thermohaline- and/or rotation-induced mixing, come into action. The extent of the influence of stellar magnetic activity on alterations in stellar chemical composition is among the least studied questions.Aims. To investigate how magnetic activity influences mixing in the atmospheres of magnetically active stars, we carried out a detailed study of C, N, and up to ten other chemical element abundances, as well as carbon isotope ratios in a sample of RS CVn stars.Methods. Using a differential model atmosphere method, we analysed high-resolution spectra that had been observed with the VUES spectrograph on the 1.65 m telescope at the Moletai Astronomical Observatory of Vilnius University. Carbon abundances were derived using the spectral synthesis of the C_{2 band heads at 5135 and 5635.5 Å. We analysed the wavelength intervals 6470–6490 Å and 7980–8005 Å with CN features to determine nitrogen abundances. Carbon isotope ratios were determined from the ^{13CN line at 8004.7 Å. Oxygen abundances were determined from the [O I] line at 6300 Å. Abundances of other chemical elements were determined from equivalent widths or spectral syntheses of unblended spectral lines.Results. We determined the main atmospheric parameters and abundances of up to 12 chemical elements for a sample of 20 RS CVn giants that represented different evolutionary stages. We determined that *29 Dra, *b01 Cyg, and V* V834 Her, which are in the evolutionary stage below the red giant branch luminosity bump, already show evidence of extra-mixing in their lowered carbon isotope ratios.Conclusions. We provide observational evidence that in low-mass chromospherically active RS CVn stars, due to their magnetic activity, extra-mixing processes may start acting below the luminosity bump of the red giant branch.}}