Most of the extrasolar planets actually known are detected from the periodic variations of 
the radial velocity of the central star. The orbital elements and mass parameters are 
obtained from a non-linear least squares fit. Although the fitting process it seems to be 
simple, in practice there are many difficulties like the short time span of observations, 
large number of parameters, the possibility of various local minimums, non Gaussian 
distribution of errors, etc.

We show the preliminary results from our orbital fittings in HD73526, HD83943, HD128311 and 
55Cnc, most of them with 2/1 mean motion resonances. The orbital fits were made combining 
genetic algorithms and simulated annealing. The uncertain were estimated with a resampling 
and Montecarlo Markov Chains which allows us to detect correlations between orbital 
parameters and non-Gaussian errors.

The results obtained were compared whit other authors and with numerical simulations 
including planetary migrations.