Revisiting the Kepler non-Blazhko RR Lyrae sample: cycle-to-cyle variations and additional modes

We analysed the long-and short-cadence light curves of the Kepler non-Blazhko RRab stars. We prepared the Fourier spectra, the Fourier amplitude and phase variation functions, time-frequency representation, the O-C diagrams and their Fourier contents. Our main findings are: (i) All stars which are brighter than a certain magnitude limit show significant cycle-to-cycle light-curve variations. (ii) We found permanently excited additional modes for at least one-third of the sample and some other stars show temporarily excited additional modes. (iii) The presence of the Blazhko effect was carefully checked and identified one new Blazhko candidate but for at least 16 stars the effect can be excluded. This fact has important consequences. The cycle-to-cycle variation phenomenon is independent from the Blazhko effect and the Blazhko incidence ratio is still much lower (51-55 per cent) than the extremely large (>90 per cent) ratio published recently. The connection between the extra modes and the cycle-to-cycle variations is marginal.