Analysis of the spurious 15-day RV signal on HD109200. Top panel: S-index time series. The star exhibits an irregular magnetic cycle with a sudden outburst of activity in 2011. The inner panel is a zoom on the 2011 season where the 36-day rotational period is visible. Middle panel: YV2 RV time series. Each observational season has been mean-centred to remove the long trend signal. The inner panel shows a zoom on the 2011 season for which the 15-day signal is clear. Bottom panel: GLS periodogram of the 2011 and 2014 season (red curve) and of the remaining time series (black curve). The 15-day signal with an amplitude of K = 1.0 m s⁻¹ is exclusively coming from those two seasons.