We are much indebted to Dr. J. Lub, Mr. K. Weerstra and Mr. L. Maitimo, who were responsible for various parts of the automatic data reduction. We like to acknowledge the following observers who made additional observations (in chronological order): R.J.L.H. Breukers, L. de Lange, P. Goudfrooy, J.J. Prein, E.W. van der Grift, H. Wever, M. Heemskerk, I. Wanders, I. Larsen, H. Kraakman, D. Heynderickx, Th. Augusteijn, R. Kalter, G. Hadiyanto Nitihardjo, B.P.M. van Esch, H. Greidanus, H.P.J. Linders, E. Kuulkers, F.H.A. Robijn, R. van der Heiden, R.A. Reijns, R.S. le Poole, O.M. Kolkman, R.L.J. van der Meer, J.M. Smit, J.P. de Jong, F.J. Dessing, G.C. Fehmers, A.M. Janssens, M.J. Zijderveld, F.C. van den Bosch and M.A.W. Verheijen.
C.S. acknowledges a research grant from the Belgian Fund for Scientific Research (FWO). This work made use of the STARLINK network. MdG thanks DENI and PPARC for support.
We are grateful to Dr. A. Gautschy for invaluable correspondence on the instability problems of massive stars, and to Dr. A. Kaufer, the referee, for his thoughtful comments that helped improve the manuscript.
© European Southern Observatory (ESO) 1998
Online publication: August 17, 1998