  The transiting extrasolar planet XO-3b is remarkable, with a high mass and
eccentric orbit. The unusual characteristics make it interesting to test
whether its orbital plane is parallel to the equator of its host star, as it is
observed for other transiting planets. We performed radial velocity
measurements of XO-3 with the SOPHIE spectrograph at the 1.93-m telescope of
Haute-Provence Observatory during a planetary transit, and at other orbital
phases. This allowed us to observe the Rossiter-McLaughlin effect and, together
with a new analysis of the transit light curve, to refine the parameters of the
planet. The unusual shape of the radial velocity anomaly during the transit
provides a hint for a nearly transverse Rossiter-McLaughlin effect. The
sky-projected angle between the planetary orbital axis and the stellar rotation
axis should be lambda = 70 +/- 15 degrees to be compatible with our
observations. This suggests that some close-in planets might result from
gravitational interaction between planets and/or stars rather than migration
due to interaction with the accretion disk. This surprising result requires
confirmation by additional observations, especially at lower airmass, to fully
exclude the possibility that the signal is due to systematic effects.
