We wish to thank the staff of the Special Astrophysical Observatory of the Russian Academy of Sciences for the granted observing time at the 6-m telescope, Dr. Panchuk V.E., Dr. Klochkova V.G. and Dr. Bikmaev I., for their contribution to the observations, and Dr. Galazutdinov G.A., for his assistance in the treatment of the images. We are especially thankful to the anonymous referee for several useful remarks on earlier version of this paper and his careful English checking. This work has been partially supported by ESO's C&EE Programme (grant A-01-004). One of the authors (GUK) wishes to express his sincere thanks to the American Astronomical Society for financial support of this work.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998