Lectures given at the Nordic-Baltic Research Course in Applied Astrophysical Photometry, held September 1999 at the Moletai Observatory, Lithuania.
The paper is available as electronic preprint astro-ph/0004049. A version with all figures in high resolution is available from the authors' home institutions at the following URLs:
Author comment submitted 20-Jun-2000 by Bo Milvang-Jensen (milvang@astro.ku.dk).