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 \fd 19 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 .