Upper panel: autocorrelation function of the full power spectrum in frequency space. The autocorrelation of the original power spectrum is plotted in black, the one of the power spectrum after prewhitening the 6 strongest peaks is plotted in grey, and then one of the 252 significant frequencies given an artificial power while all others given zero power in red. Lower panel: autocorrelation functions of the power spectrum between 0.5   d^-1 and 2   d^-1, converted into period space (colours have the same meaning). Candidate spacings are marked with a thick vertical line using a colour according to the corresponding function.