We carried out low resolution spectroscopic observations in the wavelengthrange 3400-4700 A of 20 He-weak and 8 He-strong stars to determine theirfundamental parameters by means of the Divan-Chalonge-Barbier (BCD)spectrophotometric system. For a few He-weak stars we also estimate theeffective temperatures and the angular diameters by integrating absolute fluxesobserved over a wide spectral range. Non-LTE model calculations are carried outto study the influence of the He/H abundance ratio on the emergent radiation ofHe-strong stars and on their Teff determination. We find that the effectivetemperatures, surface gravities and bolometric absolute magnitudes of He-weakstars estimated with the BCD system and the integrated flux method are in goodagreement between each other, and they also agree with previous determinationsbased on several different methods. The mean discrepancy between the visualabsolute magnitudes derived using the Hipparcos parallaxes and the BCD valuesis on average 0.3 mag for He-weak stars, while it is 0.5 mag for He-strongstars. For He-strong stars, we note that the BCD calibration, based on stars inthe solar environment, leads to overestimated values of Teff. By means of modelatmosphere calculations with enhanced He/H abundance ratios we show that largerHe/H ratios produce smaller BD which naturally explains the Teffoverestimation. We take advantage of these calculations to introduce a methodto estimate the He/H abundance ratio in He-strong stars. The BD of HD 37479suggests that the Teff of this star remains fairly constant as the starspectrum undergoes changes in the intensity of H and He absorption lines. Datafor the He-strong star HD 66765 are reported for the first time.
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