TESS sectors 3, 4 (top), and 30, 31 (bottom) PDCSAP light curves of TOI-406. In each figure, we show the best-fitting transit and Matérn-3/2 kernel GP model derived in Sect. 6 with a solid dark red line (top panel). The flattened light curve is shown in the central panel, after removing the GP component, with the solid red line showing the best-fitting model. The bottom panel shows the light curves residuals. The transits of planets b and c are highlighted with vertical orange and blue lines, respectively.