Top panel: time series of Gl 393. The source of the archival data is denoted by the colour coding. Bottom panel: generalised Lomb-Scargle periodogram of the time series of Gl 393. The dashed vertical lines denote the periods of the activity cycle (P ≈ 1226 d) and stellar rotation (P ≈ 36.6 d) determined from the time series. FAP levels of 10⁻³ and 10⁻⁹ are displayed as horizontal lines.