ESO is gratefully acknowledged for granting a 2-months studentship to SSL in 1995, during which parts of the data reduction were carried out. This research was supported by the Danish Natural Science Research Council through research grants and through its Centre for Ground-Based Observational Astronomy. We thank the anonymous referee for useful suggestions and comments which helped improve the paper.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001