I am particularly indebted to Dr Giusa Cayrel de Strobel who shared some of her very nice observations with me. I also thank Dr K. Eriksson for providing me with the GBEN grid of model atmospheres and Gerlind Brandès for kindly correcting the awkwardness of my English. This paper owes much to the corrections, constructive comments and suggestions by the referee, Dr J.J. Drake.
© European Southern Observatory (ESO) 2000
Online publication: December 17, 1999