Rendong Nan is supported by a grant from the National Natural Science Foundation in China. He also acknowledges the hospitality by the NRAO in Socorro, the JIVE institute and NAO during his visits, and thanks Dr. Athol Kemball and Dr. Scott Aaron for their useful suggestions on reduction of these data. We would also like to thank the referee for both prompt and useful comments.
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999