[1] Netherlands Inst Radio Astron, ASTRON, NL-7990 AA Dwingeloo, Netherlands
[2] Leiden Univ, Leiden Observ, NL-2300 RA Leiden, Netherlands
[3] Univ Groningen, Kapteyn Astron Inst, NL-9700 AV Groningen, Netherlands
[4] Univ Sheffield, Dept Phys & Astron, Sheffield S3 7RH, S Yorkshire, England
[5] Univ Concepcion, Dept Astron, Concepcion, Chile
[6] Ctr Astrobiol INTA CSIC, E-28850 Madrid, Spain