  In 2004, McLaughlin et al. discovered a phenomenon in the radio emission of
PSR J0737-3039B (B) that resembles drifting sub-pulses. The repeat rate of the
sub-pulses is equal to the spin frequency of PSR J0737-3039A (A); this led to
the suggestion that they are caused by incidence upon B's magnetosphere of
electromagnetic radiation from A. Here we describe a geometrical model which
predicts the delay of B's sub-pulses relative to A's radio pulses. We show that
measuring these delays is equivalent to tracking A's rotation from the point of
view of an hypothetical observer located near B. This has three main
astrophysical applications: (a) to determine the sense of rotation of A
relative to its orbital plane; (b) to estimate where in B's magnetosphere the
radio sub-pulses are modulated and (c) to provide an independent estimate of
the mass ratio of A and B. The latter might improve existing tests of
gravitational theories using this system.
