Observed minus calculated (O-C) diagram of the transit times of planet c. The estimates from K2 (blue symbols) are compared to our updated ephemeris, which uncertainties are represented by the light blue region. The predicted times of the ephemeris from HU21 (green curve) and A22 (yellow curve and region) are also shown. The full duration of the CHEOPS visit 201 is marked with the vertical dashed curve and the corresponding fitted mid-time assuming a transit detection (see Sect. 3.2) is marked with the red symbol.