Model of the SED of WASP-121 at the distance of the planet (light blue). The effective area of the instrument-filter combinationand its multiplication with the stellar spectrum are depicted in arbitrary units. An atmospheric transmission model is depicted in black and contributions of individual ions are shown above – all remaining lines are caused by Fe II. Ranges covered by HST COS observations of WASP-12 are shaded. All spectra are shown at a resolution of 3000, the nominal resolution of the CUTE cubesat (Fleming et al. 2018).