We report the detection of the primary transit of the extra-solar planet HD80606b, thanks to photometric and spectroscopic observations performed at the Observatoire de Haute-Provence, simultaneously with the CCD camera at the 120-cm telescope and the SOPHIE spectrograph on the 193-cm telescope. We observed the whole egress of the transit and partially its central part, in both datasets with the same timings. The ingress occurred before sunset so was not observed. The full duration of the transit was between9.5 and 17.2h. The data allows the planetary radius to be measured()and other parameters of the system to be refined. Radial velocity measurements show the detection of a prograde Rossiter-McLaughlin effect, and provide a hint of a spin-orbit misalignment. If confirmed, this misalignment would corroborate the hypothesis that HD80606b owes its unusual orbital configuration to Kozai migration.HD80606b is by far the transiting planet on the longest period detected today. Its unusually small radius reinforces the observed relationship between the planet radius and the incident flux received from the star and opens new questions for theory.Orbiting a bright star (V=9), it opens opportunities for numerous follow-up studies. Key words: stars: planetary systems - techniques: photometric - techniques: radial velocities - stars: individual: HD80606
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