Understanding the photometric variability of {\zeta} Ori Aa
  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. 
B. Buysschaert, C. Neiner, T. Ramiaramanantsoa, N.D. Richardson, A.   David-Uraz, A.F.J. Moffat (BEST)
astro-ph.SR