Left panel: phased light curves of WASP-121 during three transits observed with Swift UVOT labeled by their mid-transit times in BJD. The data are rebinned to 200 s intervals and corrected for the modeled systematic offsets. The uncorrected data are shown in Fig. A.1. The transit model with 1σ uncertaintyinterval of the out-of-transit flux level and transit depth is shown. The optical transit depth is depicted bythe dashed line (B-filter, Delrez et al. 2016). The observed transit depth is given at the bottom with 1σ error. Right panel: posterior of the radius ratio. Optical and IR values are indicated (Delrez et al. 2016; Evans et al. 2016)together with the expected NUV transit depth according to Rayleigh scattering.