Top panel: period–amplitude (Bailey) diagram of the OGLE bulge RRab sample (gray 2D histogram, shown on a logarithmic scale) and the mean-magnitude-changing RRL variables with circles colored according to the measured I-band mean-magnitude changes. Middle panel: distribution of epoch-independent phase differences (ϕ₃₁) of the RRab stars. Bottom panel: dependency of the amplitude ratio r_I, V on the amplitude of the I-band mean-magnitude changes. RRab stars are marked with circles and are colored according to their pulsation periods, while the single RRc star is marked with a black square. The horizontal dashed lines indicate the expected extinction ratios for different R_V values of the standard interstellar extinction law (Cardelli et al. 1989).