The author is indebted to Prof. U.Heber for his encouragement and for making available the observations used in this study. The computer programs and data used have been made available through the UK PPARC's Collaborative Computational Project No. 7. Software and computers provided by STARLINK have also been used. Peter Hamill and Sandra Jeffers acknowledge the Armagh Observatory for funding through the summer studentship programme. Financial support from DENI is gratefully acknowledged.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998