Generalized Lomb–Scargle periodograms of the SAP (top) and PDCSAP (bottom) light curves of TOI-789 after masking all transits. The red dashed line marks the candidate’s period (19.74 days), the orange dotted line the 28 day signal, and the gray dotted line twice the TESS orbital period (13.7 days); the green line indicates the 1% false-alarm level. Neither light curve shows significant power at 19.74 days. The SAP periodogram is dominated by low-frequency power spanning ∼25–70 days (peaking near 62 days; stellar rotation and its harmonics plus systematics), strongly suppressed in the PDCSAP periodogram, supporting a planetary origin for the candidate.