This research has made use of several databases and abstract services: Centre de Données Stellaires de Strasbourg (CDS), The General Catalogue of Photometric Data (GCPD, University of Lausanne), Astronomical Data Analysis Center (ADAC, NAO Japan) and NASA's Astrophysics Data System Abstract Service (ADS). Criticisms of Pr. J. Dachs on preceding versions of the paper are acknowledged as they encouraged substantial improvements. We are grateful to Dr. E. Simonneau for advice on radiation transfer simplifications used in the paper and to Drs. J. De Freitas Pachéco, L. Houziaux and Ph. Stee for fruitful discussions. Many thanks are due to A. Garcia for preparing the figures.
© European Southern Observatory (ESO) 1999
Online publication: August 25, 1999