We thank Huib Henrichs, Jan van Paradijs, Coen Schrijvers, Hiromoto Shibahashi, and Christoffel Waelkens for discussions and helpful suggestions. We thank Pieter Veen for a preprint and for providing the UV data used for Fig. 3.
This research is supported by the Netherlands Foundation for Research in Astronomy (NFRA) with financial aid from the Netherlands Organization for Scientific Research (NWO), under project 781-71-043 (JHT).
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998