CCD photometric search for peculiar stars in open clusters - I. NGC 2169, Melotte 105 and NGC 6250

The search for chemically peculiar (CP) stars in open clusters using photoelectric photometry sampling the presence of the characteristic flux depression feature at 5200 Angstrom via the Deltaa-system (Maitzen 1976) has so far delivered data for objects usually no more distant than 1000 pc from the Sun. A series of fourteen papers (first: Maitzen & Hensberge 1981; for the time being last: Maitzen 1993) were devoted to 1240 stars in 38 open cluster fields. 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. We have therefore initialized in 1995 a new survey in open clusters and the Large Magellanic Cloud using the CCD technology. As a first step, we have presented new Deltaa-photometry of 22 CP2 stars in the galactic field to prove the capability of CCD photometry for our aim (Maitzen et al. 1997). In the first paper of a new series devoted to CCD photometry, we present data on NGC 2169 (13 stars investigated), Melotte 105 (114 stars), and NGC 6250 (48 stars). NGC 2169 was used to test our results with those of classical photometry which yields excellent agreement. For NGC 6250 we find two new definite CP2 (according to the definition by Preston 1974) stars (Deltaa = 0.065 and 0.026 mag) and two lambda Bootis candidates. Twelve objects with only marginally peculiar Deltaa-values for Melotte 105 were detected. Additional spectroscopic and photometric evidence is needed to substantiate their peculiarity.