Proceedings of the International Astronomical Union (2005),
2004:4748 Cambridge University Press Nowadays, more than one
hundred extra-solar planets are known, and about a dozen of
multi-planetary systems have been discovered. Most of them have
been detected by the radial velocity (RV) method. The recovery
of orbital parameters from RV data leads to several problems.
Usually RV data cover irregularly a short time interval which is
frequently shorter than the orbital period of the most distant
planet. Moreover, observations contain a noise due to the
instabilities of the star. The distribution of this noise is
unknown. A precise determination of the dynamical state of a
multi-planetary system is important for understanding its
stability and evolution. In most cases observers determine the
orbital parameters for multi-planetary systems just fitting a
sum of Keplerian orbits. The parameters obtained in such a way
are in most cases the only accessible data about an extra-solar
system because the observes very rar