  Transit events of extrasolar planets offer a wealth of information for
planetary characterization. However, for many known targets, the uncertainty of
their predicted transit windows prohibits an accurate scheduling of follow-up
observations. In this work, we refine the ephemerides of 21 Hot Jupiter
exoplanets with the largest timing uncertainty. We collected 120 professional
and amateur transit light curves of the targets of interest, observed with 0.3m
to 2.2m telescopes, and analyzed them including the timing information of the
planets discovery papers. In the case of WASP-117b, we measured a timing
deviation compared to the known ephemeris of about 3.5 hours, for HAT-P-29b and
HAT-P-31b the deviation amounted to about 2 hours and more. For all targets,
the new ephemeris predicts transit timings with uncertainties of less than 6
minutes in the year 2018 and less than 13 minutes until 2025. Thus, our results
allow for an accurate scheduling of follow-up observations in the next decade.
