Many observations of late-type M stars show large near-spherical circumstellar envelopes, yet planetary nebulae and supernova remnants are frequently axisymmetric. We present VLBI and MERLIN observations of masers around the red supergiant S Per which show varying degrees of axisymmetry and a dynamically significant magnetic field. There is no evidence for rotation here or in most similar objects. We examine possible origins of the magnetic field.
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