The authors thank M. Orlandini for his help in analyzing the PDS data and the referee (J.Keohane) for very constructive comments. The work of A.M.B was supported by the INTAS 96-0390 and INTAS-ESA 99-1627 grants. F.B. acknowledges an ESA Research Fellowship.
© European Southern Observatory (ESO) 2000
Online publication: October 24, 2000