Discovery of a new group of double-periodic RR Lyrae stars in the OGLE-IV photometry

We report the discovery of a new group of double-periodic RR Lyrae stars from the analysis of the Optical Gravitational Lensing Experiment - IV (OGLE-IV) Galactic bulge photometry. In 11 stars identified in the OGLE catalogue as first overtone pulsators (RRc stars), we detect additional longer period variability of low amplitude, in the mmag regime. One additional star of the same type is identified in a published analysis of the Kepler space photometry. The period ratio between the shorter first overtone period and a new, longer period lies in a narrow range around 0.686. Thus, the additional period is longer than the expected period of the undetected radial fundamental mode. The obvious conclusion that addition periodicity corresponds to a gravity or a mixed mode faces difficulties, however.