  As part of our ongoing effort to investigate transit timing variations (TTVs)
of known exoplanets, we monitored transits of the four exoplanets HAT-P-18b,
HAT-P-19b, HAT-P-27b/WASP-40b and WASP-21b. All of them are suspected to show
TTVs due to the known properties of their host systems based on the respective
discovery papers. During the past three years 46 transit observations were
carried out, mostly using telescopes of the Young Exoplanet Transit Initiative.
The analyses are used to refine the systems orbital parameters. In all cases we
found no hints for significant TTVs, or changes in the system parameters
inclination, fractional stellar radius and planet to star radius ratio.
However, comparing our results with those available in the literature shows
that we can confirm the already published values.
