Top: r′ light curve of four contiguous 52.4 s data segments with no overlap (covering the orbital phase interval 2.5 to 2.535 with no interruption). Middle: mean pulse shape. The QPO light curves are folded at the best sinusoidal period indicated and repeated twice for clarity. Bottom: corresponding power spectra using Leahy normalisation. The 90% significance level is at a power value of 17. The Y-scale for power spectra is the same for all segments except the third.