Context.The study of open cluster metallicities helps to understand the local stellar formation and evolution throughout the Milky Way. Its metallicity gradient is an important tracer for the Galactic formation in a global sense. Because open clusters can be treated in a statistical way, the error of the cluster mean is minimized. Aims. Our final goal is a semi-automatic statistical robust method to estimate the metallicity of a statistically significant number of open clusters based on Johnson BV data of their members, an algorithm that can easily be extended to other photometric systems for a systematic investigation. Methods. This method incorporates evolutionary grids for different metallicities and a calibration of the effective temperature and luminosity. With cluster parameters (age, reddening and distance) it is possible to estimate the metallicity from a statistical point of view. The iterative process includes an intrinsic consistency check of the starting input parameters and allows us to modify them. We extensively tested the method with published data for the Hyades. Results. We selected sixteen open clusters within 1000 pc around the Sun with available and reliable Johnson BV measurements. In addition, Berkeley 29, with a distance of about 15 kpc was chosen. For several targets we are able to compare our result with published ones which yielded a very good coincidence (including Berkeley 29). Conclusions. A new method for the statistical determination of open cluster metallicities is presented and tested. It is quite robust against errors in effective temperature and luminosity calibration of the individual stars.

In order to put strict observational constraints on the evolutionary status of the magnetic chemically peculiar stars (CP2) of the upper main sequence, we have investigated a well established sample of galactic field CP2 objects within a radius of 200 pc from the Sun in the (X, Y) plane. In total, 182 stars with accurate parallax measurements from the Hipparcos satellite were divided into Si, SiCr and SrCrEu subgroups based on classification resolution data from the literature. Primarily, it was investigated if the CP2 phenomenon occurs at very early stages of the stellar evolution, significantly before these stars reach 30% of their life-time on the main sequence. This result is especially important for theories dealing with stellar dynamos, angular momentum loss during the pre-as well as main sequence and stellar evolutionary codes for CP2 stars. For the calibration of the chosen sample, the well-developed framework of the Geneva 7-color and Strömgren uvbyβ photometric system was used. We are able to show that the CP2 phenomenon occurs continuously at the zero age main sequence for masses between 1.5 and 4.5 M . The magnetic field strengths do not vary significantly during the evolution towards the terminal age main sequence. Only the effective temperature and magnetic field strength seem to determine the kind of peculiarity for those stars. We found several effects during the evolution of CP2 stars at the main sequence, i.e. there are two "critical" temperatures where severe changes take place. There is a transition between Si, SiCr and SrCrEu stars at 10 000 K whereas a significant decrease to almost zero of evolved SrCrEu objects with masses below 2.25 M at 8000 K occurs. These conclusions have to be incorporated into models that simulate the stellar formation and evolution of stars between 1.5 and 4.5 M in the presence of strong magnetic fields.

Abstract. We present further evidence that the magnetic chemically peculiar stars (CP2) of the upper main sequence already occur at very early stages of the stellar evolution, significantly before they reach 30% of their life-time on the main sequence. This result is especially important for models dealing with dynamo theories, angular momentum loss during the pre-as well as main sequence and evolutionary calculations for CP2 stars. Results from the literature either derived for objects in the Hyades and the UMa cluster or from the Hipparcos mission contradict each other. A way out of this dilemma is to investigate young open clusters with known ages and accurate distances (error <10%), including CP2 members. Up to now, four open clusters fulfill these requirements: IC 2391, IC 2602, NGC 2451 A and NGC 2516. In total, 13 CP2 stars can be found within these clusters. We have used the measurements and calibrations of the Geneva 7-color photometric system to derive effective temperatures and luminosities. Taking into account the overall metallicity of the individual clusters, isochrones and evolutionary tracks were used to estimate ages and masses for the individual objects. The derived ages (between 10 and 140 Myr) are well in line with those of the corresponding clusters and further strengthen the membership of the investigated CP2 stars.

We summarize of more than 25 years of research with the three filter, intermediateband, ∆a photometric system. It investigates the flux depression at λ5200 found in magnetic chemically peculiar (CP) objects. Starting with photoelectric measurements it has steadily developed introducing new and more efficient filters as well as the modern CCD technique. So far more than twenty papers were devoted to searching for new CP stars in our Milky Way up to distances of 5000 pc and even in the Large Magellanic Cloud. In the latter, the first extragalactic CP stars were detected. In addition, we have presented theoretical isochrones and synthetic colors from the latest available stellar atmospheres. The theoretical predictions agree very well with observations allowing not only to determine the reddening and age of open clusters from our photometry but also to investigate the flux depression at λ5200 in more detail.As an outlook, we present a new approach to search for chemically peculiar horizontal branch stars in globular clusters and to detect stellar variability of various objects observed during our photometric observations.

Abstract. The Michigan Catalogue (Vol. I -V) is an excellent database to study the distribution of CP2 stars in the galactic field in the Southern hemisphere. A cross-check with the BidelmanCatalogue (1973) and the "Catalogue Général desÉtoiles Ap et Am" (Renson 1991) shows that in the Michigan Catalogue 3.6% of all stars in the relevant spectral region are CP2 stars. Any discrepant classified stars could be verified photometrically in the ∆a-system. The distribution of CP2 stars does not show any particular preferred region in the Southern hemisphere.The ages of these stars and an evolutionary dependence of the CP2 effect was another important question. We conclude that the CP2 stars occupy the full Main Sequence band.