We are grateful to Drs. Viotti and Stahl for valuable comments on the results obtained. We also thank Dr. H. Lamers for critically reading the manuscript and for useful suggestions. This work was supported by NSF to Ministry of Education of Bulgaria (No. F-496/1994). Research at the Armagh Observatory is grant-aided by the Department of Education for Northern Ireland, and by the UK PPARC through the provision of the STARLINK network.
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998