  We present high precision CCD photometry of 1791 objects in 20 open clusters
with an age from 10 Myr to 1 Gyr. These observations were performed within the
Delta a photometric system which is primarily used to detect chemically
peculiar stars of the upper main sequence. Time bases range between 30 minutes
and up to 60 days with data from several nights. We describe the way of time
series analysis reaching a detection limit of down to 0.006 mag. In total, we
have detected 35 variable objects from which four are not members of their
corresponding clusters. The variables cover the entire
Hertzsprung-Russell-diagram, hence they are interesting targets for follow-up
observations.
