FIRST CCD MEASUREMENTS IN THE DELTA-A SYSTEM FOR DETECTING CP2 STARS

This is the first time that CCD-photometry in the three filter Delta a system (Maitzen 1976) was carried out after two decades of classical photoelectric photometry. The aim is to extend the successful search f or chemically peculiar stars to more and distant open clusters as well as far galactic fields. In this paper we present CCD-photometry for a sample of 25 field stars with published spectroscopic peculiarity typ es. 22 of them show discordance between their Michigan catalogue types (non-peculiar) and the Bidelman & MacConnell (1973; BC) peculiarity a ssignments. Eight stars are peculiar in Delta a (five of them also in the Geneva index Delta(V1 -G)) thus supporting BC, mainly among their Silicon star discoveries. Considering cases where Silicon line enhance ments were interpreted as peculiar instead as indicator of higher lumi nosity by BC, and the perfect detection of the three unambiguous pecul iar stars, in addition to the achievement of an accuracy of 0.003 mags , we are justified to conclude that the performance of our CCD-Delta a photometry perfectly matches the requirements of the conventional pho tometric technique and thus opens the way to observations of fainter s tars.