GLS periodograms of the original TERRA (a), DRS (b), and R20 (c) RVs and their residuals after recursive pre-whithening. For each figure: the vertical and dashed blue lines indicate the stellar rotation frequency and its first and second harmonic; the red line marks the orbital frequency of the planet announced by Donati et al. (2016, 2017); each panel reports the RMS of the dataset used for calculating a specific periodogram; the window function is shown in green in the bottom panel.