Overview of the images used for the CCD photometry of NGC4103 and NGC5281. Overview of the images used for the CCD photometry of NGC4755. Parameters used for the calibration of the photometric data of NGC4103, NGC5281, and NGC4755. The parameters are defined by Eqs. The field of view is .
North is up and east to the left.
Fields B and C were observed to provide a sufficiently large area which does not contain cluster stars, which is necessary for the statistical field star subtraction. Transformation of instrumental to apparent magnitudes was performed for field C, which was observed under photometric conditions. Typical internal errors of stars in different magnitude ranges are given in Table 4 . The entire photometric data are listed in Table 5 . The lack of bright red stars already shows at this point that NGC4103 is a young star cluster.