Petersen diagram for additional unknown signals in studied RRc stars. We used red diamonds to indicate if the additional detected signal has a period longer than the first-overtone period and black squares if otherwise. We also plotted RR_0.61 and RR_0.68 stars identified in this study using purple circles. For comparison, we plotted known multiperiodic groups: RRd stars (green points; Soszyński et al. 2019), fundamental and second overtone (F+2O) stars (yellow pluses; Jurcsik et al. 2008; Poretti et al. 2010; Chadid et al. 2010; Benkő et al. 2010, 2014), peculiar RRd (pRRd) stars (cyan triangles; Prudil et al. 2017), and RR_0.61 and RR_0.68 from the Galactic bulge (orange crosses and blue diamonds, respectively; Netzel & Smolec 2019).