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We present new observations of four closely-spaced NUV transits of the hotJupiter-like exoplanet WASP-12b using HST/COS, significantly increasing thephase resolution of the observed NUV light curve relative to previousobservations, while minimising the temporal variation of the system. We observesignificant excess NUV absorption during the transit, with mean normalisedin-transit fluxes of $F_\mathrm{norm}\simeq0.97$, i.e. $\simeq$2-5 $\sigma$deeper than the optical transit level of $\simeq0.986$ for a uniform stellardisk (the exact confidence level depending on the normalisation method used).We further observe an asymmetric transit shape, such that the post-conjunctionfluxes are overall $\simeq$2-3 $\sigma$ higher than pre-conjunction values, andcharacterised by rapid variations in count rate between the pre-conjunction andout of transit levels. We do not find evidence for an early ingress to the NUVtransit as suggested by earlier HST observations. However, we show that the NUVcount rate observed prior to the optical transit is highly variable, butoverall $\simeq$2.2-3.0 $\sigma$ below the post-transit values and comparablein depth to the optical transit, possibly forming a variable region of NUVabsorption from at least phase $\phi\simeq$0.83, limited by the data coverage.

Hubble Space Telescope observations of the NUV transit of WASP-12b