  Photometric variability of chemically peculiar (CP) stars of the upper main
sequence is closely connected to their local stellar magnetic field and their
rotational period. Long term investigations, as presented here, help us to
identify possible stellar cycles (as in the Sun). Furthermore, these data
provide a basis for detailed surface mapping techniques. Photoelectric
Stroemgren uvby time series for 27 CP stars within the boundaries of open
clusters are presented. In addition, Hipparcos photometric data (from 1989 to
1993) are used for our analysis. Our observations cover a time period of about
six years (1986 to 1992) with typically fifteen measurements for each objects.
These observations help us to determine the rotational periods of these
objects. A standard reduction procedure was applied to the data. When possible,
we merged our data sets with already published ones to obtain a more
significant result. A detailed time series analysis was performed, involving
five different methods to minimize spurious detections. We established, for the
first time, variability for fourteen CP stars. For additional two stars, a
merging of already published data sets, resulted in more precise periods,
whereas for six objects, the published periods could be confirmed. Last, but
not least, no significant variations were found for five stars. Apart from six
stars, all targets seem to be members of their host open clusters.
