Additional modes and cycle-to-cycle variations of non-Blazhko RR Lyrae stars

This paper presents our study on the Kepler non-Blazhko RR Lyrae sample. We analysed long and short cadence Kepler flux curves with different methods such as studying the Fourier spectra, the Fourier amplitude and phase variation functions, the O−C diagrams and their Fourier contents. We detected significant cycle-to-cylce variations of the short cadence flux curves. Noise or instrumental origin of the effect has been checked and excluded. In addition, we detected low amplitude additional modes for six stars (a third of the sample). Two stars proved to be anomalous RRd stars containing the radial first overtone frequency while the other four ones show the second radial overtone frequency and its linear combinations. The presence of the Blazhko effect was also tested. We found only one possible Blazhko star in this sample which confirms the ∼ 50% Blazhko incidence ratio for the Kepler RRab sample