GLS periodograms, computed from the offset-corrected values. The upper panel shows the raw periodogram of the RVs as shown in Fig. 2. The second panel shows the periodogram after the removal of the long-term trends (removed with a simple second-degree polynomial regression for pre-whitening purposes only). The lower panels show the periodograms after the removal of the Keplerian signals of HD 15337 b and c from the previous one. Vertical and horizontal lines as Fig. 2.