We want to express our gratitude to R. Wielebinski and A. Jessner for their constant help, support and valuable advices. We thank M. Timofeev for help with the observations and D. Lorimer for his helpful comments and discussions. We are also in particular grateful to C. Salter for extremely valuable suggestions and thank an anonymous referee for helpful comments. Arecibo Observatory is operated by Cornell University under cooperative agreement with NSF. BJR acknowledges the support from the NSF under grant AST-9414144.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998