1. Centro de Investigaciones de Astronomı́a, Apartado Postal 264, Mérida 5101-A, Venezuela.
2. Department of Astronomy, Yale University, New Haven, CT 06511, USA.
3. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA.
4. Physics Department, Universidad de los Andes, Mérida 5101, Venezuela.
5. Department of Physics, Yale University, New Haven, CT 06520, USA.