Maitzen (1976) introduced a 3 filter system with intermediate
band width for measuring the depth of the broad flux depression around 5200
Å, which was shown to occur among the magnetic peculiar stars. In the
division proposed by Preston (1974) these objects constitute
the whole CP2 and part of the CP4 group (Helium weak stars). Maitzen
& Vogt (1983) have substantiated the original finding and definitely
established with a very large sample of stars that the photometric index 
is a very efficient means for identifying magnetic CP-stars, enabling
statistical studies in a non-subjective way and to much larger distances
than the traditional spectroscopic detection method would allow.
The non-magnetic branch of CP-stars comprising CP1 (Am stars) and CP3
(Hg Mn stars) did not show individually peculiar values of
albeit as groups they deviated from normal stars in the 
range. This value was based on rather low number statistics and
called for better foundation. This is one of the goals of the present
investigation.
A second aim of this work is to contribute to the Renson (1991)
catalogue of Ap and Am stars (6684 objects)
which lists also Ap-detections obtained by the
system. In cases where spectroscopic evidence had been published, our
values can give additional hints for the assessment of a star
to be peculiar in view of the
probability for peculiarity as given by Renson.
This probability was indicated by him using four different symbols:
* well established
blank very probable
? less probable
erroneous.
In this connection it should be mentioned that already the very first "catalogue" of peculiar A stars - a list of 9 stars - brought to the public by Maury (1897) hundred years ago contains one erroneous classification as Silicon star (81 UMa) which has not been confirmed by subsequent publications.
While the number of very probably erroneous peculiarity classifications in the Renson catalogue is of the order of a few percent, the number of objects with a question mark comprises more than 30 percent and further evidence on the nature of these objects, e.g. by our photometry seems to be desirable.
This is also valid for those stars in the Renson catalogue which do not exhibit a proper CP1-4 type, but just a HD classification without direct indication of the reason why they were included in the catalog.
A further goal will be the comparison with the list of values
published by Lebedev (1986), in which also spectrophotometric
data were used to synthesize .
Helpful and clarifying will be the comparison with the peculiarity parameters of the Geneva system derived from the catalogue of Rufener (1988).
Since about 20 percent of our programme stars lack photometry in the
Strömgren system (which we need for deriving values)
we have included such measurements.
As a consequence, we have measured both standard and programme stars
in 6 filters: uvbyg1g2, using a novel photometer with a rapidly rotating
filter wheel in stop and go motion.
This instrumentation has been devised to enable photometric measurements also
under non-excellent weather conditions.