V.M.ade is grateful to the 'Onderzoeksfonds K.U.Leuven' for awarding him the senior research fellowship (F/97/9). Á. Csík is grateful to the Hungarian Soros Foundation, the "Onderzoeksfonds K.U.Leuven" and to the Hungarian Airlines Company for the financial support. The authors are also thankful to M.Ruderman for valuable discussions on the considered subject.
© European Southern Observatory (ESO) 1998
Online publication: September 30, 1998