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We have used the MOST (Microvariability and Oscillations of STars)microsatellite to obtain four weeks of contiguous high-precision broadbandvisual photometry of the O7.5III(n)((f)) star {\xi} Persei in November 2011.This star is well known from previous work to show prominent DACs (DiscreteAbsorption Components) on time-scales of about 2 d from UV spectroscopy and NRP(Non Radial Pulsation) with one (l = 3) p-mode oscillation with a period of 3.5h from optical spectroscopy. Our MOST-orbit (101.4 min) binned photometry failsto reveal any periodic light variations above the 0.1 mmag 3-sigma noise levelfor periods of hours, while several prominent Fourier peaks emerge at the 1mmag level in the two-day period range. These longer-period variations areunlikely due to pulsations, including gravity modes. From our simulations basedupon a simple spot model, we deduce that we are seeing the photometricmodulation of several co-rotating bright spots on the stellar surface. In ourmodel, the starting times (random) and lifetimes (up to several rotations) varyfrom one spot to another yet all spots rotate at the same period of 4.18 d, thebest-estimated rotation period of the star. This is the first convincingreported case of co-rotating bright spots on an O star, with importantimplications for drivers of the DACs (resulting from CIRs - CorotatingInteraction Regions) with possible bright-spot generation via a breakout at thesurface of a global magnetic field generated by a subsurface convection zone.

MOST detects corotating bright spots on the mid-O type giant {\xi} Persei