[1] Consejo Nacl Invest Cient & Tecn, Inst Astron & Fis Espacio, RA-1428 Buenos Aires, DF, Argentina
[2] Observ Paris, Sect Meudon, DASOP, URA2080, F-92195 Meudon, France
[3] Univ Oslo, Inst Theoret Astrophys, N-0315 Oslo, Norway
[4] Univ Birmingham, Birmingham B15 2TT, W Midlands, England
[5] NASA, George C Marshall Space Flight Ctr, Huntsville, AL 35812 USA