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Transit timing variations (TTVs) of exoplanets are normally interpreted asthe consequence of gravitational interaction with additional bodies in thesystem. However, TTVs can also be caused by deformations of the system transitsby starspots, which might thus pose a serious complication in theirinterpretation. We therefore simulate transit light curves deformed byspot-crossing events for different properties of the stellar surface and theplanet, such as starspot position, limb darkening, planetary period, and impactparameter. Mid-transit times determined from these simulations can besignificantly shifted with respect to the input values; these shifts cannot belarger than ~1% of the transit duration and depend most strongly on thelongitudinal position of the spot during the transit and the transit duration.Consequently, TTVs with amplitudes larger than the above limit are veryunlikely to be caused by starspots. We also investigate whether TTVs fromsequences of consecutive transits with spot-crossing anomalies can bemisinterpreted as the result of an additional body in the system. We use theGeneralized Lomb-Scargle periodogram to search for periods in TTVs and concludethat low amplitude TTVs with statistically significant periods around activestars are the most problematic cases. In those cases where the photometricprecision is high enough to inspect the transit shapes for deformations, itshould be possible to identify TTVs caused by starspots, however, especiallyfor cases with low transit signal to noise light curves (TSNR $\lesssim$ 15) itbecomes quite difficult to reliably decide whether these periods come fromstarspots, physical companions in the system or if they are random noiseartifacts.

How do starspots influence the transit timing variations of exoplanets? Simulations of individual and consecutive transits