1. Instituto de Astronomía, Universidad Nacional Autónoma de México, A. P. 70-264, C.P. 04510, México, D.F., Mexico
2. Department of Astronomy, University of Wisconsin Madison, 475 N. Charter Street, Madison, WI 53706, USA
3. Apache Point Observatory and New Mexico State University, PO Box 59, Sunspot, NM 88349-0059, USA
4. Sternberg Astronomical Institute, Moscow State University, Moscow, Russia