We would like to thank Françoise Combes and the referee (Linda Sparke) for helpful comments and criticism. VR acknowledges support from French Minist re de la Recherche et de la Technologie during his stay in Paris. This work was supported by grant N 94-02-06026-a from Russian Foundation for Basic Research.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998