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Up to now, it seemed that the additional frequencies in the fundamental mode(RRab) and in the overtone mode pulsating (RRc and RRd) RR Lyrae stars havedifferent nature. RRab stars show frequencies associated with periodicdoubling, as well as frequencies at the first and second radial overtones, andlinear combinations of these. RRc stars show frequencies with specific ratios($f_1/f_x\sim$0.61, or $\sim$0.63) which are explained by non-radial modes andfrequencies with $f_x/f_1\sim0.68$ ratio which have no currently acceptedexplanation. To search for similarities in spectral content, we compared theFourier spectra of the recently published TESS and K2 data of RRc stars withthe spectra of Kepler RRab stars that do not show the Blazhko effect butcontain additional frequencies. The time series data have been analysed usingstandard Fourier methods, and the possibility of the excitation of the secondradial overtone mode in RRab stars has also been tested using numericalhydrodynamical codes. We show that the additional frequencies appear innon-Blazhko RRab stars at the position of the second radial overtone mode, andthe pattern they create, is very similar to that caused by the additionalfrequencies with the period ratio $\sim0.68$ in RRc stars. The formerexplanation of the additional frequencies of these RRab stars by a secondradial overtone is unlikely.

Similar additional frequency patterns on fundamental and overtone mode RR Lyrae stars showing $f_{68}$ frequencies