GLS periodograms of activity indexes (from top to bottom panels): Sindex (HIRES+HARPS+HARPS-N); Hα and NaD (HARPS+HARPS-N) and window function. The 15.5 d period is indicated in each periodogram, as the red dashed line. The significant periods discussed in Sect. 3.4, are indicated with a magenta dot. In the Hα periodogram (HARPS+HARPS-N), we also indicated, with a blue dot, the 40.8 d period which becomes significant in the residual time series, after the removal of the long 2500 d period.