M. Ollé thanks P. Comas for his help in all computer and software issues, and is very grateful to the Geneva Observatory, where this work was done. We thank G. Contopoulos and D. Friedli for interesting comments on the manuscript. The first author has been partially supported by DGICYT grant no. PR95-240 and CIRIT grants no. 1995BEAI400002 and GRQ93-1135. This work was also supported by the Swiss National Science Foundation (SNSF). The Povray team is acknowledged for its free ray-tracing software used for Fig. 3.
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998