The constructive suggestions by F.P. Pijpers and A. Weiss have significantly improved the paper. Discussions with W. Glatzel, B. Gustafsson, L.-A. Willson and P. Woitke have been very helpful. The author is grateful to L. Achmad, H.J.G.L.M. Lamers, N. Langer and S. Owocki for communicating their work prior to publication.
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998