Discovery of Peculiar Double-Mode Pulsations and Period Doubling in KEPLER RRc Variables

We analyzed the Long Cadence (29.4 min) photometry of four first overtone RR Lyrtype stars (RRc stars) observed by the Kepler telescope. Only Q0, Q1 and Q2 datasets were used, spanning 138 days. Analysis including the remaining Kepler data will be discussed elsewhere (Moskalik et al. 2012, in preparation). The frequency analysis was conducted with the standard consecutive prewhitening technique (see e.g. [3]). All four RRc variables turned out to be multiperiodic. Fig. 1 shows the prewhitening sequence for KIC 5520878. After removing the dominant frequency f1 = 3.715 c/d and its harmonics, the Fourier transform of the residuals (middle panel) is dominated by a strong peak at f2 = 5.879 c/d, its harmonic and their combinations with the primary frequency, f1. The period ratio of the two modes is P2/P1 = 0.6319. After prewhitening the data with both f1, f2 and their