O–C plot showing the deviations in transit time from the best-fit linear ephemeris (grey dashed line) for KELT-9 b. The transit number is shown on the x-axis. The y-axis shows the difference in observed and calculated mid-transit time. The orange line shows the best orbital decay fit to the KELT-9 b TTV data, with the red line showing the best apsidal precession fit. CHEOPS data are highlighted in black.