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 \sim 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 ( \mbox { $v \sin { i } $ } = 19.6 ^ { +2.2 } _ { -2.1 } km s ^ { -1 } ) compared to the value found from spectroscopic line broadening ( \mbox { $v \sin { i } $ } = 13.4 \pm 1.5 km s ^ { -1 } ) .
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 designed to address this issue , we find v \sin { i } to be consistent with the value obtained from spectroscopic broadening measurements ( \mbox { $v \sin { i } $ } = 15.7 ^ { +1.4 } _ { -1.3 } km s ^ { -1 } ) .