Near-infrared period–luminosity relations for RRab and RRc stars (left) and all RRL stars (right) in J (top), H (middle), and K_s (bottom). In the right panels, the periods for the RRc stars have been shifted to their corresponding fundamental-mode periods, as explained in the text. The dashed lines represent best-fitting linear regressions over the period range under consideration, while the dotted lines display ±2.5σ offsets from the best-fitting PLRs. The shaded (gray) line and crossed (brown) lines represent theoretically predicted PLRs for metallicities representative of NGC 6441 but with canonical and enhanced helium, respectively.