An extensive $\Delta a$-photometric survey of southern B and A type bright stars

Photoelectric photometry of 803 southern BS objects in the a-system as detection tool for magnetic chemically peculiar (=CP2) stars has been carried out and compared to published spectral types. The statistical yield of such objects detected by both techniques is practically the same. We show that there are several factors which contaminate the search for these stars, but this contami- nation is only of the order of 10% in both techniques. We nd a smooth transition from normal to peculiar stars. Our sample exhibits the largest fraction of CP2 stars at their bluest colour interval, i.e. 10% of all stars in the colour range 0:19 B V< 0 :10 or 0:10 b y< 0 :05. No peculiar stars based on the a-criterion were found at bluer colours. Towards the red side the fraction of CP2 stars drops to about 3% for positive values of B V or b y with red limits roughly corresponding to normal stars of spectral type A5. The photometric behaviour of other peculiar stars: Am, HgMn, Del, Boo, He abnormal stars, as well as Be/shell stars and supergiants shows some slight, but def- inite deviations from normal stars. Spectroscopic and vi- sual binaries are not distinguished from normal stars in their a behaviour. The results of this work justify larger statistical work (e.g. in open clusters) employing more time-saving photo- metric methods (CCD).