QPO frequency profiles. Mean power density spectra of the three light curves (top: r′, middle: He, bottom: u′) for the full ~5.6 h observation are shown in logarithmic scale in the range 0.03−9.76 Hz. The Y scale is the squared fractional amplitude in units of rms² Hz^-1. Two best fit power laws are shown representing the additional low-frequency red noise (dotted line) and high-frequency white noise (dashed line); their sum (full line) has been subtracted to compute the QPO power (see text).