PRIMORDIAL HELIUM AND DELTA-Y DELTA-Z FROM H-II REGIONS AND FROM FINE-STRUCTURE IN THE MAIN-SEQUENCE BASED ON HIPPARCOS PARALLAXES/

The primordial helium abundance Y-P is important for cosmology and the ratio Delta Y/Delta Z of the changes relative to primordial abundance s constrains models of stellar evolution. While the most accurate esti mates of Y-P come from emission lines in extragalactic H II regions, t hey involve an extrapolation to zero metallicity which itself is close ly tied up with the slope Delta Y/Delta Z. Recently certain systematic effects have come to light in this exercise which make it useful to h ave an independent estimate of Delta Y/Delta Z from fine structure in the main sequence of nearby stars. We derive such an estimate from Hip parcos data for stars with Z less than or equal to Z. and find values between 2 and 3, which are consistent with stellar models, but still h ave a large uncertainty.