This work was supported by the TMR programme of the European Union under contract FMRX-CT98-0168. It was initiated during a visit by LD to the Service d'Astrophysique, CEA Saclay, whose hospitality is gratefully acknowledged. We wish to thank M. Cassé and Elisabeth Vangioni-Flam for stimulating discussions of these and related topics, and Keith Olive for suggestions improving the presentation of the ideas.
© European Southern Observatory (ESO) 1999
Online publication: May 6, 1999