The authors thank Dr.T.Blöcker for providing detailed data on his recent evolutionary tracks for CSPN. The authors are grateful to the referee Dr.G.Mellema for criticism and valuable suggestions. SZh acknowledges the Visiting Professor programme of GNA-CNR at Osservatorio Astrofisico di Arcetri, Firenze, Italy where part of this work has been done. This work was also partially sponsored by the National Science Fund of the Bulgarian Ministry of Education, Science and Technoligies under contract F-570.
© European Southern Observatory (ESO) 1998
Online publication: May 12, 1998