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Hi Peter,
These are dust doughnuts. Caused by very small bits of dust on the surface of either one of your filters (if you're using one) or the glass at the front of the CCD.
They can be cleaned off relatively easily. One of the best ways to get rid of them is called an Arctic Butterfly, see if you can get one, they're worth every cent you spend on one.
Flats will help to remove them, electronically (and any vignetting in your system), but it's better to remove them, physically.
Cheers
Stuart
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