Study of Chi² when fitting the EPiCA time series with a three-planet model. Left: Iso-contours of Chi² as a function of planets c and d time lag. For each pair of planet c and d time lag with BJD_ref, a best fit of the time lag of planet b is computed, as well as the overall Chi² on the 1151 points. The minimum Chi² is at 3014, the average Chi² per point is 2.6 (ms⁻¹)². Top right: Horizontal cross section of left image, with the variation of the time lag of planet b, and Chi², as a function of the time lag of planet c, when the time lag of planet d is fixed to its overall best-fit value. Bottom right: Same as top, but the roles of planet d and c are inverted. For both panels, the variation of the time lag of planet b is very small, showing an absence of coupling between the phases (or time lags) of the three planets.