  The Transit Timing Variations (TTVs) are deviations of the measured
mid-transit times from the exact periodicity. One of the most interesting
causes of TTVs is the gravitational interaction between planets. Here we
consider a case of two planets in a mean motion resonance (orbital periods in a
ratio of small integers). This case is important because the resonant
interaction can amplify the TTV effect and allow planets to be detected more
easily. We develop an analytic model of the resonant dynamics valid for small
orbital eccentricities and use it to derive the principal TTV terms. We find
that a resonant system should show TTV terms with two basic periods (and their
harmonics). The resonant TTV period is proportional (m/M_*)^(-2/3), where m and
M_* are the planetary and stellar masses. For m=10^(-4) M_*, for example, the
TTV period exceeds the orbital period by ~2 orders of magnitude. The amplitude
of the resonant TTV terms scales linearly with the libration amplitude. The
ratio of the TTV amplitudes of two resonant planets is inversely proportional
to the ratio of their masses. These and other relationships discussed in the
main text can be used to aid the interpretation of TTV observations.
