Light curves of the stellar flux (top) extracted from the simulated images of two transits of an Earth-size planet in front of a Sun-like star (case 1). The two transits are phase-folded. The black points are the 60-min binned data and the red curve corresponds to the best-fit model. The regular data gaps visible in the series are related to pointing visibility limits of CHEOPS during its orbit. Time is expressed from mid-transit. Phase-folded residuals (bottom).