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tokens: of magnetic variability. We demonstrate here that using multiband photometric light curves contemporaneous to transit observations, we can overcome this problem, at least partly. The advantage of using several photometric bands is that, as discussed in the previous section (see also Mallonn et al. 2018 ), it is possible to independently estimate the zero point with respect to the unspotted photosphere ( z ), and thus the absolute spot filling factor (including any persistent level of spottedness during the rotation cycle), and the temperature contrast. This provides extremely valuable information to correct transit depth variations due to spots. We can actually use our StarSim simulations of WASP-52 to investigate at which level of accuracy we can correct the effects of starspots on transit data. The StarSim simulations of the WASP-52 planetary system discussed in the previous section reveal that, without additional information allowing us to infer the distribution and filling factor of spots during the transit, the planet-to-star radius cannot be determined with an accuracy better than a few percent. This is because in that case, we do not know at which rotation phase of the star the transit is observed, and therefore, we can only estimate a range of possible filling factors based on the amplitude of the photometric modulation. However, if we know the rotation phase at the time of transit, and we can estimate the spot properties and distribution from light curves, we can infer the correction that needs to be applied to the relative radius of the exoplanet (or transit depth) with an accuracy of order 10 −4 , as shown in Table 4 . Figure 10 illustrates the sequence of spot corrections on the transit depth as a function of time. We plot the planet radius variation due to unnoculted spots, Δ r SP ∕ R * , following our StarSim model describing the photometric light curve in Fig. 8 . Each panel corresponds to a different wavelength band of interest for the Ariel mission, as an example. To test all possible filling factors, we consider transits occurring at any time, but actual WASP-52 b transits are indicated as filled dots. The average value of all the simulations performed is displayed as a solid line. It is clear from this figure that the variation of the apparent planet radius depends on the projected spot filling factor and follows the stellar rotation period. The relative effect on the radius diminishes towards longer wavelengths due to the lower flux contrast of spots, from an average of ~4 × 10 −3 to ~8 × 10 −4 at 550 nm (VIS-Phot) and 6 μ m (AIRS-Ch1), respectively. In all cases it is possible to estimate the unocculted spot correction to the planetary relative radius with a precision close to 10%, estimated as the standard deviation ( σ ) of the sample of 21 optimal fits to the light curves. As explained above, this residual uncertainty is due to the variance of the stellar parameters and the randomness of the determination of the active