  We studied the variability of the magnetic O-type supergiant $\zeta$ Ori Aa
using multi-colour BRITE photometry. We confirmed the known rotation frequency
$f_{\rm rot} = 0.15 \pm 0.02$ c/d, and detected some of its higher harmonics,
of which $4f_{\rm rot}$ is compatible with the known DAC recurrence timescale.
Thanks to simultaneous high-resolution CHIRON spectroscopy, we could identify
another frequency $f_{\rm env} = 0.10 \pm 0.02$ c/d, caused by the
circumstellar environment. Variations in the circumstellar environment are
believed to cause the observed difference between the BRITE lightcurves.

Solar and Stellar Astrophysics