We have used the MOST ( Microvariability and Oscillations of STars ) microsatellite to obtain four weeks of contiguous high-precision broadband visual 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 ( Discrete Absorption 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 3.5  h from optical spectroscopy .
Our MOST -orbit ( 101.4 Â min ) binned photometry fails to reveal any periodic light variations above the 0.1 Â mmag 3 -sigma noise level for periods of hours , while several prominent Fourier peaks emerge at the 1 Â mmag level in the two-day period range .
These longer-period variations are unlikely due to pulsations , including gravity modes .
From our simulations based upon a simple spot model , we deduce that we are seeing the photometric modulation of several co-rotating bright spots on the stellar surface .
In our model , the starting times ( random ) and lifetimes ( up to several rotations ) vary from one spot to another yet all spots rotate at the same period of 4.18 Â d , the best-estimated rotation period of the star .
This is the first convincing reported case of co-rotating bright spots on an O star , with important implications for drivers of the DACs ( resulting from CIRs - Corotating Interaction Regions ) with possible bright-spot generation via a breakout at the surface of a global magnetic field generated by a subsurface convection zone .