This is the proceedings version of an invited talk given at the IAU Symposium 201 in Manchester in August 2000.
The links below point to one file containing the complete text, all figures and tables etc.
As are all my publications this is available in two forms:
last modified on Saturday, December 24, 2005 at 10:25:30 PM by helbig@astro.mNuOlStPiAvMa!x.de