Bottom: two-dimensional time-frequency image of the rms fractional amplitude in a 210 s time interval for the r′ filter (same interval as in Fig. 11). The X-axis is the orbital phase and the Y-axis is the frequency in the restricted range 0−1 Hz, increasing from top to bottom. Colour-coded rms amplitude is shown according to the scale at right (in units of 10^-6 rms² Hz^-1). Sliding 52.4 s FFTs have been computed with a 1.28 s step. Top: corresponding fractional r′ light curve shown after normalisation by dividing the original data by a 5 s moving average and after a 0.5 s smoothing to better show the shape of oscillations.