The authors thank Anatoly Tur for helpful discussions, useful comments of an anonymous referee are greatly acknowledged. I.M. is grateful to the SIGMA team of CESR for warm hospitality and facilities which made his stay in Toulouse very fruitful and pleasant. This work was partly supported through a post-doctoral fellowship of the French Ministry of Research.
© European Southern Observatory (ESO) 1997
Online publication: May 5, 1998