1. Kapteyn Astronomical Institute, University of Groningen, 9700AV Groningen, The Netherlands;
2. Departement Cassiopee, Université de Nice Sophia-Antipolis, Observatoire de la Côte d'Azur, CNRS, F-06304 Nice Cedex 4, France;
3. INAF-Osservatorio Astronomico di Bologna, I-40127 Bologna, Italy;