[1] Osserv Astron Capodimonte, INAF, I-80131 Naples, Italy
[2] Univ Calif Santa Cruz, UCO Lick Observ, Santa Cruz, CA 95064 USA
[3] Univ Zurich, Inst Theoret Phys, CH-8057 Zurich, Switzerland
[4] Univ Napoli Federico II, Dipartimento Sci Fisiche, I-80126 Naples, Italy