Searching for transit timing variations in the known transiting
exoplanet systems can reveal the presence of other bodies in the system.
Here we report such searches for two transiting exoplanet systems,
TrES-1 and WASP-2. Their new transits were observed with the 4.2m
William Herschel Telescope located on La Palma, Spain. In a continuing
programme, three consecutive transits were observed for TrES-1, and one
for WASP-2 during September 2007. We used the Markov Chain Monte Carlo
simulations to derive transit times and their uncertainties. The
resulting transit times are consistent with the most recent ephemerides
and no conclusive proof of additional bodies in either system was found.