  Despite the existence of co-orbital bodies in the solar system, and the
prediction of the formation of co-orbital planets by planetary system formation
models, no co-orbital exoplanets (also called trojans) have been detected thus
far. Here we study the signature of co-orbital exoplanets in transit surveys
when two planet candidates in the system orbit the star with similar periods.
Such pair of candidates could be discarded as false positives because they are
not Hill-stable. However, horseshoe or long libration period tadpole co-orbital
configurations can explain such period similarity. This degeneracy can be
solved by considering the Transit Timing Variations (TTVs) of each planet. We
then focus on the three planet candidates system TOI-178: the two outer
candidates of that system have similar orbital period and had an angular
separation near $\pi/3$ during the TESS observation of sector 2. Based on the
announced orbits, the long-term stability of the system requires the two
close-period planets to be co-orbitals. Our independent detrending and transit
search recover and slightly favour the three orbits close to a 3:2:2 resonant
chain found by the TESS pipeline, although we cannot exclude an alias that
would put the system close to a 4:3:2 configuration. We then analyse in more
detail the co-orbital scenario. We show that despite the influence of an inner
planet just outside the 2:3 mean-motion resonance, this potential co-orbital
system can be stable on the Giga-year time-scale for a variety of planetary
masses, either on a trojan or a horseshoe orbit. We predict that large TTVs
should arise in such configuration with a period of several hundred days. We
then show how the mass of each planet can be retrieved from these TTVs.
