Transiting exoplanets orbiting active stars frequently occult starspots and faculae on the visible stellar disc. Such occultations can provide useful constraints to retrieve the temperature of active features and their effect on transmission spectra. We analyse the constraints that the James Webb Space Telescope will be able to place on the spectra of starspots, despite its lack of optical wavelength instruments on board. Focusing on K and M spectral types, we find that starspot temperatures can be determined to within a few hundred kelvins using NIRSpec/Prism and the proposed NIRCam/F150W2+F322W2’s broad wavelength capabilities.

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GB acknowledges support from CHEOPS ASI-INAF agreement n. 2019-29-HH.0. GM and GC acknowledge the support of the ARIEL ASI-INAF agreement n.2018-22-HH.0. This research has made use of the SVO Filter Profile Service (http://svo2.cab.inta-csic.es/theory/fps/) supported from the Spanish MINECO through grant AYA2017-84089.