  We present an observation of the Rossiter-McLaughlin effect for the planetary
system WASP-3. Radial velocity measurements were made during transit using the
SOPHIE spectrograph at the 1.93m telescope at Haute-Provence Observatory. The
shape of the effect shows that the sky-projected angle between the stellar
rotation axis and planetary orbital axis (lambda) is small and consistent with
zero within 2 sigma; lambda = 15 +10/-9 deg. WASP-3b joins the ~two-thirds of
planets with measured spin-orbit angles that are well aligned and are thought
to have undergone a dynamically-gentle migration process such as planet-disc
interactions. We find a systematic effect which leads to an anomalously high
determination of the projected stellar rotational velocity (vsini = 19.6
+2.2/-2.1 km/s) compared to the value found from spectroscopic line broadening
(vsini = 13.4 +/- 1.5 km/s). This is thought to be caused by a discrepancy in
the assumptions made in the extraction and modelling of the data. Using a model
developed by Hirano et al. (2009) designed to address this issue, we find vsini
to be consistent with the value obtained from spectroscopic broadening
measurements (vsini = 15.7 +1.4/-1.3 km/s).
