Context. With the ∆a photometric system, it is possible to study very distant galactic and even extragalactic clusters with a high level of accuracy. This can be done with a classical color-magnitude diagram and appropriate isochrones. The new calibration presented in this paper is a powerful extension.Aims. For open clusters, the reddening is straightforward for an estimation via Isochrone fitting and is needed in order to calculate the reddening-free, temperature sensitive, index (g 1 − y) 0 . As a last step, the calibration can be applied to individual stars. Methods. Because no a priori reddening-free photometric parameters are available for the investigated spectral range, we have applied the dereddening calibrations of the Strömgren uvbyβ system and compared them with extinction models for the Milky Way. As expected from the sample of bright stars, the extinction is negligible for almost all objects. As a next step, already established calibrations within the Strömgren uvbyβ, Geneva 7-color, and Johnson UBV systems were applied to a sample of 282 normal stars to derive a polynomial fit of the third degree for the averaged effective temperatures to the individual (g 1 − y) 0 values. Results. We derived an empirical temperature calibration for the ∆a photometric system for A-type to mid F-type with a mean of the error for the whole sample of ∆T eff is 134 K, which is lower than the value in Paper I for hotter stars. No statistically significant effect of the rotational velocity on the precision of the calibration was found. Conclusions. We have derived a new intrinsically consistent, empirical, effective temperature calibration for a spectral range from early B-type to mid F-type, luminosity class V to III stars within the photometric ∆a system. The statistical mean error over the complete spectral range of about 140 to 240 K will allow to individual objects of far distant galactic be studied as well as extragalactic clusters with high accuracy.

We establish an empirical effective temperature calibration of main sequence, luminosity class V to III B-type stars for the ∆a photometric system which was originally developed to detect magnetic chemically peculiar objects of the upper main sequence (early B-type to early F-type) at 5200 Å. However, this system provides the index (g 1 − y) which shows an excellent correlation with (B − V) as well as (b − y) and can be used as an indicator of the effective temperature. This is supplemented by a very accurate color-magnitude diagram, y or V versus (g 1 − y), which can be used, for example, to determine the reddening, distance and age of an open cluster. This makes the ∆a photometric system an excellent tool to investigate the Hertzsprung-Russell-Diagram (HRD) in more detail. Using the reddening-free parameters and already established calibrations within the Strömgren uvbyβ, Geneva 7-color and Johnson UBV systems, a polynomial fit of third degree for the averaged effective temperatures to the individual (g 1 − y) 0 values was derived. For this purpose, data from the literature as well as new observations were taken resulting in 225 suitable bright normal B-type objects. The statistical mean of the error for this sample is 238 K which is sufficient to investigate the HRD of distant galactic open clusters as well as extragalactic aggregates in the future.

We analyze light curves of the W UMa type eclipsing binary V781 Tauri from three epochs and radial velocity curves from two epochs simultaneously, including previously unpublished B and V data. The overall time span is from 1983 to 2000 and the solution is done coherently in time (not phase) with five light curves and two sets of primary and secondary velocity curves. Minor systematic differences among the individual light curves are not large enough to undermine the value of a coherent solution that represents 18 years of observations. Times of minima confirm a period of 0.d 34491 and the general solution finds a small period change, dP/P, of (5.08 ± 04) × 10 −11 that represents recent behavior. The eclipse timings cover the last half-century and find dP/P about four times smaller, corresponding to a period change time scale, P/(dP/dt) of about 6 million years. The system is over-contact with a filling factor of 0.205. The solution produces a temperature difference of about 260 K between the components, an inclination of 65.• 9, and a mass ratio M 2 /M 1 = 2.47. Separate solutions of the several light curves that incorporate dark spots find parameters that differ little from curve to curve. Absolute masses, luminosities, radii and the distance are derived, with luminosities and distance based on star 1 being of type G0V. The orbital angular momentum is compared with those of other W UMa type binaries and is normal. The star to star mass flow that one can infer from dP/dt is opposite to that expected from TRO (Thermal Relaxation Oscillator) theory, but pertains to a time span that is very short compared to the time scale of TRO oscillations.

Context. The chemically peculiar (CP) stars of the upper main sequence are mainly characterized by strong overabundances of heavy elements. Two subgroups (CP2 and CP4) have strong local magnetic fields which make them interesting targets for astrophysical studies. This star group, in general, is often used for the analysis of stellar formation and evolution in the context of diffusion as well as meridional circulation. Aims. In continuation of a long term study of CP stars (initiated in the 1980s), we present new results based on photoelectric measurements for ten open clusters that are, with one exception, younger than 235 Myr. Observations in star clusters are favourable because they represent samples of stars of constant age and homogeneous chemical composition. Methods. The very efficient tool of Δa photometry was applied. It samples the flux depression at 5200 Å typically for CP stars. In addition, it is able to trace emission line Be/Ae and λ Bootis stars. Virtually all CP2 and CP4 stars can be detected via this tool, and it has been successfully applied even in the Large Magellanic Cloud. For all targets in the cluster areas, we performed a kinematic membership analysis. Results. We obtained new photoelectric Δa photometry of 304 stars from which 207 objects have a membership probability higher than 50%. Our search for chemically peculiar objects results in fifteen detections. The stars have masses between 1.7 M and 7.7 M and are between the zero-and terminal-age-main-sequence. We discuss the published spectral classifications in the light of our Δa photometry and identify several misclassified CP stars. We are also able to establish and support the nature of known bona fide CP candidates. Conclusions. It is vital to use kinematic data for the membership determination and also to compare published spectral types with other data, such as Δa photometry. There are no doubts about the accuracy of photoelectric measurements, especially for stars brighter than 10th magnitude. The new and confirmed CP stars are interesting targets for spectroscopic follow-up observations to put constraints on the formation and evolution of CP stars.

Nearly 250 close visual binaries have been observed using the area scanner photometer since 1973. The observations were carried out at Figl-Observatory, Austria, Mauna Kea Observatory, Hawaii, and European Southern Observatory, Chile. Through the generous help of the Director of the Institute for Astronomy in Honolulu, Prof. Dr. John T. Jefferies, the 24 inch telescope on Mauna Kea was available for this program. Also the Directorate of ESO has kindly made available the 100 cm and 50 cm telescopes for this purpose.