FUNDAMENTAL STELLAR PARAMETERS FOR NEARBY VISUAL BINARY STARS - ETA-CAS, EPSILON-BOO, 70 OPH AND 85 PEG - HELIUM ABUNDANCE, AGE AND MIXING LENGTH PARAMETER FOR LOW-MASS STARS

The calibration of nearby visual binary stars on the HR diagram is use d to determine the helium abundance, age and mixing length parameter f or stars other than the Sun. Four Population I low mass systems with h igh-quality data are analysed by means of standard evolutionary stella r models: eta Gas, 70 Oph, xi Boo and 85 Peg.Complementing these resul ts with those for the Sun and alpha Cen it is shown that in the framew ork of the mixing-length formalism to describe convection, a unique va lue of the mixing-length parameter, equal to the solar one can be used to model these objects. Except for the 85 Pegasi system, which cannot be explained by means of standard stellar evolutionary models, the he lium abundance is determined with a precision of 0.02 (p.e.) and the a ge with a precision of 2 Gyr (p.e.). A concomitant positive relation b etween metallicity and helium abundance is found for these stars, corr esponding to a mean value of about Delta Y/Delta Z approximate to 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 mi ssion are briefly discussed.