  We present a new analysis of the dynamics of the planetary system around the
pulsar B1257+12. A semi-analytical theory of perturbation between
terrestrial-mass planets B and C is developed and applied to improve
multi-orbit timing formula for this object. We use numerical simulations of the
pulse arrival times for PSR B1257+12 to demonstrate that our new timing model
can serve as a toll to determine the masses of the two planets.
