![]() | ![]() |
Astron. Astrophys. 358, 886-896 (2000) AcknowledgementsThis research was partly supported by the Deutsche Forschungsgemeinschaft. We thank also the anonymous referee for useful comments and suggestions which helped us to improve the clarity of the paper. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 2000 Online publication: June 20, 2000 ![]() |