Nice one Vincent. In my experience it can be a result of the guiding star you pick and slight field rotation (worst if you guide from a star right in the corner of the sensor)
or your 'flat' field is not centered on your CCD. You should be able to pick that up by shooting one flat and looking at the illumination. I bet the bright spot would be at 9'0clock on your shot. Hyperstar is tricky to get right. Myself I never get it consistently in the same spot.
