1. Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
2. Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
3. Astronomical Institute Anton Pannekoek, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands