The census of non-radial pulsation in first-overtone RRLyrae stars of the OGLE Galactic bulge collection

We analysed photometry for the up-to-date collection of the first-overtone RRLyrae stars (RRc; 11415 stars) and double-mode RRLyrae stars (RRd; 148 stars) towards the Galactic bulge from the Optical Gravitational Lensing Experiment (OGLE). We analysed frequency spectra of these stars in search for additional, low-amplitude signals, beyond the radial modes. We focused on stars from two groups: RR0.61 and RR0.68. In the first group, additional low-amplitude signals have periods shorter than the first-overtone period; period ratios fall in the 0.60-0.64 range. In the second group, additional low-amplitude signals have periods longer than the first-overtone period; period ratios tightly cluster around 0.68. Altogether we have detected 960 and 147 RRLyrae stars that belong to RR0.61 and RR0.68 groups, respectively, which yield the incidence rates of 8.3 and 1.3percent of the considered sample. We discuss statistical properties of RRLyrae stars with additional periodicities. For RR0.61 group we provide strong arguments that additional periodicities are connected to non-radial pulsation modes of degrees l=8 and l=9, as proposed by Dziembowski. We have also detected two double-periodic variables, with two close periodicities, similar to RRLyrae variable V37 in NGC6362. Properties of these peculiar variables, which may form a new group of double-mode pulsators, are discussed.