O–C plot showing the deviations in transit time from the best-fit linear ephemeris (grey dashed line) for KELT-16 b. The transit number is shown on the x-axis, and the timing difference on the y-axis. The orange line shows the best quadratic fit to the KELT-16 b TTV data, with the red line showing the best apsidal precession fit. CHEOPS data are highlighted in black. The data from the literature are described in more detail in Sect. 2.4.2.