  We report on the first measurement of the Eu isotope fractions (151Eu and
153Eu) in s-process-element-enhanced, metal-poor stars. We use these ratios to
investigate the 151Sm branching of s-process nucleosynthesis. The measurement
was made by detailed study of Eu II lines that are significantly affected by
hyperfine splitting and isotope shifts in spectra of the carbon-rich, very
metal-poor stars LP625-44 and CS31062-050, observed with the Subaru Telescope
High Dispersion Spectrograph. The 151Eu fractions [fr(151Eu) =
151Eu/(151Eu+153Eu)] derived for LP625-44 and CS31062-050 are 0.60 and 0.55,
respectively, with uncertainties of about +/- 0.05. These values are higher
than found in solar-system material, but agree well with the predictions of
recent s-process models. We derive new constraints on the temperature and
neutron density during the s-process based on calculations of pulsed s-process
models for the 151Eu fraction.
