We have analyzed new and archival time series spectra taken six years apart
during transits of the hot Jupiter WASP-33 b, and spectroscopically resolved
the line profile perturbation caused by the Rossiter-McLaughlin effect. The
motion of this line profile perturbation is determined by the path of the
planet across the stellar disk, which we show to have changed between the two
epochs due to nodal precession of the planetary orbit. We measured rates of
change of the impact parameter and the sky-projected spin-orbit misalignment of
db/dt=−0.0228−0.0018+0.0050 yr−1 and
dλ/dt=−0.487−0.076+0.089~∘ yr−1, respectively,
corresponding to a rate of nodal precession of
dΩ/dt=0.373−0.083+0.031~∘ yr−1. This is only the
second measurement of nodal precession for a confirmed exoplanet transiting a
single star. Finally, we used the rate of precession to set limits on the
stellar gravitational quadrupole moment of
9.4×10−5<J2<6.1×10−4.Comment: Published in ApJL. 5 pages, 3 figures. Corrected error in the
calculation of J_
