An empirical temperature calibration for the

We establish an empirical effective temperature calibration of main sequence, luminosity class V to III B-type stars for the $\Delta a$ 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 $\Delta a$ 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 $uvby\beta$, 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