Bayesian generalized Lomb–Scargle periodograms of the blue (top panel), red (middle panel), and HARPS RVs and their difference (bottom panel). The three dashed vertical lines depict the orbital period of the planets K2-18b and c (P_c ~ 9 days), and the photometric stellar rotation period. The 9-day signal is seen in the first two time-series, but at a lower probability in the blue likely due to the higher levels of stellar activity in that wavelength regime. The 9-day signal is suppressed in the GLSP of the RV difference, while some residual probability close to P_rot continues topersist due to the incomplete removal of stellar activity.