SIMBAD references

The calibration of nearby visual binary stars on the HR diagram is used to determine the helium abundance, age and mixing length parameter for stars other than the Sun. Four Population I low mass systems with high-quality data are analysed by means of standard evolutionary stellar models: η Cas, 70Oph, ξ Boo and 85Peg. Complementing these results with those for the Sun and α Cen it is shown that in the framework of the mixing-length formalism to describe convection, a unique value of the mixing-length parameter, equal to the solar one can be used to model these objects. Except for the 85Pegasi system, which cannot be explained by means of standard stellar evolutionary models, the helium abundance is determined with a precision of 0.02 (p.e.) and the age with a precision of 2Gyr (p.e.). A concomitant positive relation between metallicity and helium abundance is found for these stars, corresponding to a mean value of about ΔY/ΔZ ≃3±2 (relative helium-to-heavier-elements enrichment parameter) but there is no clear correlation between age and metallicity. The consequences of the results of the use of the new parallaxes from the HIPPARCOS mission are briefly discussed.