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Abstract :
[en] Abstract : We present here the discovery by the WASP-South survey, in close collaboration with the Euler and TRAPPIST telescopes, of the transiting planet WASP-121b as well as the measurement of its thermal emission at 0.9 microns. WASP-121b is a very inflated (1.76 RJup) Jupiter-mass (1.02 MJup) planet that transits every 1.27 days a bright F6V star. It is remarkable as its orbital radius is only ~10% larger than its Roche limit, suggesting that it might experience mass loss through Roche-lobe overflow. Thanks to its large size and extreme irradiation (~7 10^9 erg s-1 cm-2), it was predicted to display a thermal emission of ~0.1% of the stellar flux in the near-infrared. Using the TRAPPIST robotic telescope, we could detect this thermal emission signal at ~5 sigma in the z'-band. This measurement, a first for a ground-based 60cm telescope, allows to place preliminary constraints on the atmospheric properties of this very special hot Jupiter.