Top row: Bayesian generalized Lomb–Scargle periodograms of the HARPS RVs in the three observing seasons annotated above each panel. The vertical dashed lines depict the orbital period of the proposed non-transiting planet K2-18c (P_c = 8.962 days), the orbital period of the known transiting planet K2-18b (P_b = 32.93963 days), and the photometric stellar rotation period (P_rot = 38.6 days). Bottom row: sodium doublet time-series as measured by HARPS. The coloured regions/markers are indicative of the epochs used to compute each RV GLSP in theupper row. The annotation group adjacent to each observing season depicts the Na D peak-to-peak amplitude A, the NaD rms, and the number of measurements within that observing season. These A and rms diagnostics indicate that the level of stellar activity is decreasing with time, while the P_c signal is simultaneously becoming more prominent.