O–C plot showing the deviations in transit time from the best-fit linear ephemeris (grey dashed line) for WASP-4 b. The transit number is shown on the x-axis, with the difference in timing being shown on the y-axis. The orange line shows the quadratic fit to the WASP-4 b transit timing variation data, and the red line shows the apsidal precession fit. CHEOPS data are highlighted in black.