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1 Introduction

Statistical studies on chemically peculiar stars of the upper main sequence (CP-stars) have been primarily based on spectroscopic detections. However, in the case of the magnetic subgroup CP2 (and the magnetic part of subgroup CP4, the Helium-weak stars) use was made of their broad band 520nm flux depression to develop photometric indices in the Geneva system ($\Delta(V1 - G)$, Hauck 1974) and in the system of Maitzen (1976, $\Delta a$) in order to detect those objects in larger samples and in an impersonal way.

The identification power of $\Delta a$ was demonstrated by Maitzen & Vogt (1983) for a sample of 339 southern CP2 stars detected spectroscopically by Bidelman & MacConnell (1973). Especially for the most numerous subgroup, the Silicon stars, the detection capacity of the $\Delta a$-index approaches 100%.

After having shown that virtually all spectroscopically detected CP2 stars are also identified by the $\Delta a$ index as peculiar, the second task to embark on has been to show that systematically positive $\Delta a$-values are only found and typical for magnetic CP-stars. In this process the case of Pleione (Maitzen & Pavlovski 1987), a B8 star with emission and shell episodes introduced a new aspect: it had a distinctly positive $\Delta a$-value in 1984 which faded to zero with fading shell phase. This means that a certain impurity for the magnetic CP detection capability of $\Delta a$ may have to be taken into account for stars similar to Pleione, hence the Be/shell stars. A current programme (Maitzen & Pavlovski 1997), however, is showing that this impurity is of minor importance since the case of Pleione is a very outstanding one, and that $\Delta a$ values among Be/shell stars are only marginally positive. It is worthwile mentioning at the same time, that those stars while in the emission phase usually show a systematically negative $\Delta a$-index.

Our present investigation which comprises the largest sample of programme stars ever measured in $\Delta a$, i.e. 803 objects, aims at:

solving the problem whether $\Delta a$-detections match the existing spectroscopic detections of magnetic CP-stars,
finding out whether there are $\Delta a$ peculiar stars with no published spectroscopic evidence of peculiarity,
studying the transition from normal to peculiar stars in terms of the $\Delta a$-index,
identifying systematic $\Delta a$-behaviour, both positive and negative for other groups of stars (e.g. Am=CP1 stars, $\lambda$ Boo stars, Be and B shell stars).

Concerning spectroscopic evidence we were hampered by the following situation: although the photometric material had been gathered about two decades ago, at that time its spectroscopic counterpart was not mature for reasonable comparison, since the spectral classifications in the old BS-catalogue were based on both very inhomogeneous material and classificators. The situation has improved due to revised spectral types in the new BS-catalogue (Hoffleit 1982), but even more so by the huge undertaking of the Michigan catalogue series started nearly two decades ago, which stands out by its homogeneous plate material and uniform classification skill (Houk & Cowley 1975; Houk 1978; Houk 1982; and Houk & Smith-Moore 1988). Though it has not been fully finished yet, we deem it justified now to proceed with the publication of our comparisons.

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