Context :

Aims : The transiting planet WASP-12 b was identified as a potential target for transit timing studies because a departure from a linear ephemeris was reported in the literature .
Such deviations could be caused by an additional planet in the system .
We attempt to confirm the existence of claimed variations in transit timing and interpret its origin .

Methods : We organised a multi-site campaign to observe transits by WASP-12 b in three observing seasons , using 0.5–2.6-metre telescopes .

Results : We obtained 61 transit light curves , many of them with sub-millimagnitude precision .
The simultaneous analysis of the best-quality datasets allowed us to obtain refined system parameters , which agree with values reported in previous studies .
The residuals versus a linear ephemeris reveal a possible periodic signal that may be approximated by a sinusoid with an amplitude of 0.00068 \pm 0.00013 d and period of 500 \pm 20 orbital periods of WASP-12 b .
The joint analysis of timing data and published radial velocity measurements results in a two-planet model which better explains observations than single-planet scenarios .
We hypothesize that WASP-12 b might be not the only planet in the system and there might be the additional 0.1 M _ { Jup } body on a 3.6-d eccentric orbit .
A dynamical analysis indicates that the proposed two-planet system is stable over long timescales .

Conclusions :