Similar additional frequency patterns on fundamental- and overtone-mode RR Lyrae stars showing f68 frequencies

Context. Until now, it has been accepted that the additional frequencies in the fundamental-(RRab) and overtone-mode pulsating (RRc and RRd) RR Lyrae stars are of a different nature. RRab stars show frequencies associated with periodic doubling, as well as frequencies at the first and second radial overtones, and linear combinations of these. RRc stars show frequencies with specific ratios ( f 1 / f x ∼ 0.61 or ∼0.63), which are explained by non-radial modes, and frequencies with a ratio of f x / f 1 ∼ 0.68, for which there is currently no accepted explanation. Aims. To search for similarities in spectral content, we compared the recently published Fourier spectra of TESS and K2 RRc stars with the spectra of Kepler RRab stars that do not show the Blazhko effect but contain additional frequencies. Methods. We analysed the time series data using standard Fourier methods, and also investigated the possibility of excitation of the second radial overtone mode in RRab stars using numerical hydrodynamical codes. Results. We show that the additional frequencies appear in non-Blazhko RRab stars at the position of the second radial overtone mode, and the pattern they create is very similar to that caused by the additional frequencies with the period ratio of ∼0.68 in RRc stars. The formerly raised hypothesis that the additional frequencies of these RRab stars are due to a second radial overtone is unlikely.