Expected photometric performance of CHEOPS. Left panel: for bright stars (here assumed to be G-type stars with an effective temperature of 5500 K and apparent magnitude 6 ≤ V <  9), the rms of the light curve residuals is required to be less than 20 ppm in six hours of integration, limited mainly by instrumental noise. Right panel: for faint stars (here assumed to be K-type stars with an effective temperature of 4500 K and apparent magnitude 9 ≤ V ≤ 12), the rms of the light curve residuals is required to be less than 85 ppm in three hours of integration, dominated by instrument noise for 9 ≤ V <  10, by photon noise for 10 ≤ V <  11 and by stray light for V ≥ 11. We note that unlike photon noise, instrument noise does not scale quadratically with the integration time.