  About two years ago, Wolszczan and Frail announced the detection of a
possible planetary system consisting of two Earth-mass planets around a
millisecond pulsar. It was pointed out shortly thereafter that the mutual
gravitational interaction of the planets leads to predictable evolution of
their orbits. Because millisecond pulsars are very stable ``clocks'' and allow
pulse timing measurements with microsecond precision, the orbit evolution would
lead to a measurable modulation of the arrival times of the pulses. Detection
of this effect would serve to corroborate the planetary interpretation of the
data and provide additional constraints on the system. The theory of planetary
perturbations in the context of pulsar timing observations is summarized here.
