Hess diagrams for two different regions of the survey. The top panel shows a region that was observed in both V and I bands in our survey. The region shown in the bottom panel was observed only in I and we used the 2MASS J band to construct a color–magnitude diagram. Black vertical lines indicate the color cut for selecting RR Lyrae star candidates (however, the most interesting region was explored with out a color cut). The triangle symbols indicate the confirmed RR Lyrae stars with a brightness similar to the horizontal branch of ω Centauri (see Sect. 4.2). Circles correspond to other RR Lyrae stars in the field of view (stars in the foreground or background of the cluster). RR Lyrae stars of types ab and c are presented as red and white symbols. The color scale is proportional to the number of stars in each bin.