  We perform a numerical study of the systematic effects involved in the
determination of the critical line at real baryonic chemical potential by
analytic continuation from results obtained at imaginary chemical potentials.
We present results obtained in theories free of the sign problem, such as
two-color QCD with finite baryonic density and three-color QCD with finite
isospin chemical potential, and comment on general features which could be
relevant also to the continuation of the critical line in real QCD at finite
baryonic density.
