CP2 stars in clusters: deep Delta a-photometry

The search for chemically peculiar (CP) stars in open clusters using photoelectric photometry sampling the presence of the characteristic flux depression feature at 5200A via the Delta a-system (Maitzen 1976) has so far delivered data for objects usually no more distant than 1000 pc from the Sun. If one intends to study the presence of CP stars at larger distances from the Sun, classical photometry has to be replaced by CCD photometry. For the first time, our investigation presents the results of CCD-photometry in the Delta a-system for a rich open cluster which is at a distance clearly beyond hitherto studied objects, Melotte 105 (2 kpc, log age = 8.5). Comparison with published uvby-photometry yields the calibration of the colour index g_1-y of our system, which is necessary for deriving the peculiarity index Delta a. For this we achieve an average accuracy of 0.007 mag. Six objects with only marginally peculiar Delta a-values were found, but spectroscopic and additional photometric evidence is needed to substantiate their peculiarity.