1. Observatoire de la Côte d'Azur, B.P. 4229, 06304 Nice Cedex 4, France.
2. Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland.
3. Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy.
4. Department of Astronomy, University of Maryland, College Park, MD 20742–2421, USA.