[1] Aalto Univ, Metsahovi Radio Observ, Metsahovintie 114, FI-02540 Kylmala, Finland
[2] Tuorla Observ, FI-21500 Piikkio, Finland
[3] Univ Nacl Autonoma Mexico, Astron Inst, Mexico City 04510, DF, Mexico
[4] Inst Nacl Astrofis Opt & Electr, Puebla 72000, Mexico
[5] MPI Radioastronomie, D-53121 Bonn, Germany