  We present new U-band photometry of the magnetic Helium-strong star Sigma Ori
E, obtained over 2004-2009 using the SMARTS 0.9-m telescope at Cerro Tololo
Inter-American Observatory. When combined with historical measurements, these
data constrain the evolution of the star's 1.19 d rotation period over the past
three decades. We are able to rule out a constant period at the p_null = 0.05%
level, and instead find that the data are well described (p_null = 99.3%) by a
period increasing linearly at a rate of 77 ms per year. This corresponds to a
characteristic spin-down time of 1.34 Myr, in good agreement with theoretical
predictions based on magnetohydrodynamical simulations of angular momentum loss
from magnetic massive stars. We therefore conclude that the observations are
consistent with Sigma Ori E undergoing rotational braking due to its magnetized
line-driven wind.
