Top: ellipsoidal variations modelled with and without secondary terms for the best-fit parameters in the CHEOPS passband. The x-axis represents the orbital phase of the planet θ = ν + ω, where ν and ω are the true anomaly and the argument of periastron, respectively. The blue curve (Sine model) is computed as a simple sine wave, i.e. with A ₁ = A ₃ = 0. The other models include the secondary terms that are computed for several values of the gravity-darkening coefficient y. Bottom: differences between the different EV models and the Sine model. The peak-to-peak difference in amplitude is of the order of 50 ppm for the realistic range of GDC values (0.1 ≤ y ≤ 0.5). The effect of accounting for A ₁ and A ₃ in the modelling of EV impacts mainly the difference in baseline level between transit and occultation. The baseline flux during transit will be lower than that during occultation (effect similar to a negative nightside flux). If not taken into account, this may lead to overestimating the occultation depth or underestimating the nightside flux from a planet.