Timing of transiting exoplanets is expected to provide discoveries of additional, even very low mass bodies in extrasolar systems. In a single-planet system, a transiting planet orbits its host star on a Keplerian orbit. If there is another planet in the system, it interacts gravitationally with the transiting planet what generates deviations from the strictly Keplerian case. These perturbations result in a quasi-periodic signal in an observed-minus-calculated (O-C) diagram of the transiting planet. The TTV method is sensitive to small perturbing masses in orbits near to the low-order mean-motion resonances. Deriving the orbital elements and mass of the perturber from the TTV signal is a difficult inverse problem. Different configurations may generate similar TTV characteristics with an identical dominant periodicity. For a given transiting planet, the TTV signal depends on perturber's mass and its orbital elements. Exploring such a multi-dimensional space of parameters is not a trivial task and systematic observations are crucial to finding a correct solution.

Data from telescopes located at various longitudes are needed to observe most target's transits which are available in an observing season. Thus, we organise multi-site campaigns which engage telescopes in Europe, Asia and America. Our strategy may be summarised as follows:

• Choosing a target sample: the most promising targets are being selected by reanalysis of available literature data. We observe only 2–3 targets in a given season to maximise the efficiency of the project.
• Reconnaissance campaign: an attempt to detect TTV with telescopes smaller than 1 m. If a TTV signal is detected, a provisional hypothesis is put forward. Models, which assume the existence of a perturber in a system and reproduce observed variation of timing residuals, are identified by three-body simulations.
• Follow-up campaign: a photometric follow-up is run with big telescopes (>1 m) to verify and characterise TTV signal. In some cases a spectroscopic follow-up is organised to get more precise radial velocity measurements. If a claimed TTV signal is confirmed, a final hypothesis is elaborated, based on both high-precision photometric and spectroscopic data.
• Direct confirmation: attempts to detect transits of a perturbing planet by photometry acquired by large telescopes.

(http://adsabs.harvard.edu/abs/2011EPJWC..1105009M)