Periodogram of RV residuals for WASP-12 b on a Keplerian orbit (continuous black line) and the spectral window (dashed red line, multiplied by 10 for clarity). The arrow indicates the strongest peak at 3.58 d that is not associated with significant power in the spectral window. The shortest period is set by the radius of the star. The dashed and dotted horizontal lines show empirical FAP levels of 1% and 0.1%, respectively.