[1] Stockholm Univ, Dept Astron, S-10691 Stockholm, Sweden
[2] Stockholm Univ, Oskar Klein Ctr, S-10691 Stockholm, Sweden
[3] Univ Groningen, Kapteyn Astron Inst, Groningen, Netherlands
[4] UCL, Dept Phys & Astron, London, England
[5] Harvard Univ, Ctr Astrophys, Cambridge, MA 02138 USA
[6] Max Plank Inst Astrophys, Garching, Germany
[7] ASTRON, Dwingeloo, Netherlands
[8] Radboud Univ Nijmegen, Dept Astron, NL-6525 ED Nijmegen, Netherlands
[9] Scuola Normale Super Pisa, Pisa, Italy
[10] Univ Sussex, Dept Phys & Astron, Brighton, Sussex, England
[11] Stockholm Univ, Dept Phys, S-10691 Stockholm, Sweden
[12] Univ Edinburgh, Inst Astron, Edinburgh, Midlothian, Scotland
[13] Univ London Imperial Coll Sci Technol & Med, Dept Phys, London, England
[14] Univ Tecn Lisboa, Inst Super Tecn, CENTRA, Lisbon, Portugal
[15] Univ Bielefeld, Fac Phys, D-33615 Bielefeld, Germany
[16] Observ Paris, F-75014 Paris, France