  The aim of this work is a detailed analysis of transit light curves from
TrES-1 and TrES-2, obtained over a period of three to four years, in order to
search for variabilities in observed mid-transit times and to set limits for
the presence of additional third bodies. Using the IAC 80cm telescope, we
observed transits of TrES-1 and TrES-2 over several years. Based on these new
data and previously published work, we studied the observed light curves and
searched for variations in the difference between observed and calculated
(based on a fixed ephemeris) transit times. To model possible transit timing
variations, we used polynomials of different orders, simulated O-C diagrams
corresponding to a perturbing third mass and sinusoidal fits. For each model we
calculated the chi-squared residuals and the False Alarm Probability (FAP). For
TrES-1 we can exclude planetary companions (>1 M_earth) in the 3:2 and 2:1 MMRs
having high FAPs based on our transit observations from ground. Additionally,
the presence of a light time effect caused by e. g. a 0.09 M_sun mass star at a
distance of 7.8 AU is possible. As for TrES-2, we found a better ephemeris of
Tc = 2,453,957.63512(28) + 2.4706101(18) x Epoch and a good fit for a sine
function with a period of 0.2 days, compatible with a moon around TrES-2 and an
amplitude of 57 s, but it was not a uniquely low chi-squared value that would
indicate a clear signal. In both cases, TrES-1 and TrES-2, we were able to put
upper limits on the presence of additional perturbers masses. We also conclude
that any sinusoidal variations that might be indicative of exomoons need to be
confirmed with higher statistical significance by further observations, noting
that TrES-2 is in the field-of-view of the Kepler Space Telescope.
