  While not detected yet, pairs of exoplanets in the 1:1 mean motion resonance
probably exist. Low eccentricity, near-planar orbits, which in the comoving
frame follow the horseshoe trajectories, are one of the possible stable
configurations. Here we study transit timing variations produced by mutual
gravitational interaction of planets in this orbital architecture, with the
goal to develop methods that can be used to recognize this case in
observational data. In particular, we use a semi-analytic model to derive
parametric constraints that should facilitate data analysis. We show that
characteristic traits of the transit timing variations can directly constrain
the (i) ratio of planetary masses, and (ii) their total mass (divided by that
of the central star) as a function of the minimum angular separation as seen
from the star. In an ideal case, when transits of both planets are observed and
well characterized, the minimum angular separation can also be inferred from
the data. As a result, parameters derived from the observed transit timing
series alone can directly provide both planetary masses scaled to the central
star mass.
