tokens: I thank Chris Nixon and Stuart Muldrew for discussions and help in preparing the Letter, and the referee for a very perceptive report. This paper was completed during a visit to the Institut d'Astrophysique, Paris, and I am grateful to them for support. I particularly thank Jean-Pierre Lasota for discussions of accretion disc stability during my stay. Theoretical astrophysics research in Leicester is supported by an STFC Consolidated Grant.
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