SIMBAD references

We present high precision CCD photometry of 1791 objects in 20 open clusters with an age of 10Myr to 1Gyr. These observations were performed within the Δa photometric system which is primarily used to detect chemically peculiar stars of the upper main sequence. Time bases range from 30min up to 60-days with data from several nights. We describe the time series analysis reaching a detection limit of down to 0.006mag for apparent variability. In total, we have detected 35 variable objects of 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.