  We report the detection of the primary transit of the extra-solar planet HD
80606 b, thanks to photometric and spectroscopic observations performed at
Observatoire de Haute-Provence, simultaneously with the CCD camera at the
120-cm telescope and the SOPHIE spectrograph at the 193-cm telescope. We
observed the whole egress of the transit and partially its central part, in
both data sets with the same timings. The ingress occurred before sunset and
was not observed. The full duration of the transit was between 9.5 and 17.2
hours. The data allows the planetary radius to be measured (Rp = 0.9 +- 0.1
RJup) 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 for a spin-orbit misalignment. If confirmed, this misalignment
would corroborate the hypothesis that HD 80606 b owes its unusual orbital
configuration to Kozai migration. HD 80606 b 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 quite bright star
(V=9), it opens opportunities to numerous follow-up studies.
