  We report on observations of the Rossiter-McLaughlin (RM) effect for the XO-3
exoplanetary system. The RM effect for the system was previously measured by
two different groups, but their results were statistically inconsistent. To
obtain a decisive result we observed two full transits of XO-3b with the Subaru
8.2-m telescope. By modeling these data with a new and more accurate analytic
formula for the RM effect, we find the projected spin-orbit angle to be
\lambda=37.3 deg \pm 3.0 deg, in good agreement with the previous finding by
Winn et al. (2009). In addition, an offset of ~22 m/s was observed between the
two transit datasets. This offset could be a signal of a third body in the XO-3
system, a possibility that should be checked with future observations. We also
attempt to search for a possible signature of the stellar differential rotation
in the RM data for the first time, and put weak upper limits on the
differential rotation parameters.
