Two-dimensional time-frequency image of the fractional rms amplitude for the r′ light curve. The X-axis is the orbital phase and the Y-axis is the frequency in the restricted range 0−5 Hz. Colour-coded squared amplitude is shown according to the scale at the right (in units of 1.4 × 10^-5 rms² Hz^-1). The individual (52.4 s) spectra have been normalised to their average level in the range 5−9 Hz and the image has been smoothed with a two-point Gaussian filter. The corresponding normalised r′ light curve is shown at the top.