1. Instituto de Física, Universidad Autónoma de Puebla, Apartado Postal J-48, Puebla 72570, Mexico
2. On sabbatical leave at Instituto Nacional de Astrofisica Optica y Electrónica, Tonantzintla, Puebla 72840, Mexico
3. National Institute of Astrophysics, Optics and Electronics, INAOE, Electronics Group, Tonantzintla, Puebla 72840, Mexico