RXTE/ASM folded light curves of GX 13+1 in the 1.3–5 keV (top panels) and 5–12.1 keV energy band (middle panels) covering about 714 days. The corresponding HRs are plotted in the bottom panels. The folding period is 24.52 d and 150 phase-bins per period were adopted. The solid red line is the best-fit curve composed of a constant, plus a sinusoidal function with its period kept fixed to 24.52 d and a Gaussian profile fitting the dip.