  High resolution (R=110000), very high S/N spectra, centered on the 6707.8A
LiI line have been obtained with the ESO Coude Echelle Spectrometer for five,
metal-poor (-0.8 < [Fe/H] < -0.6) disk stars in the turnoff region of the
HR-diagram. The instrumental and stellar atmospheric line broadening have been
determined from two unblended iron lines and used in a model atmosphere
synthesis of the profile of the LiI line as a function of the lithium isotope
ratio. This has led to a detection of 6Li in HD68284 and HD130551 with 6Li/7Li
= 0.05, whereas the other stars, HR2883, HR3578 and HR8181, have 6Li/7Li close
to zero.
  By comparing Teff-values and absolute magnitudes based on Hipparcos
parallaxes with recent stellar evolutionary tracks, the masses of the stars
have been derived. It is shown that the two stars with 6Li present have a
significantly higher mass, M = 1.05 M(sun), than the other three stars for
which values between 0.85 and 1.0 M(sun) are obtained.
  The results are discussed in terms of models for the galactic evolution of
the light elements and depletion of the lithium isotopes in stellar envelopes.
It is shown that the measured 6Li abundances are in agreement with standard
cosmic ray production of 6Li in the galactic disk and a moderate depletion (0.5
dex) in the stars. Recent models for the evolution of 6Li including alpha +
alpha fusion reactions and predicting a high lithium isotopic ratio, 6Li/7Li =
0.3 at [Fe/H] = -0.6, require a high degree of 6Li depletion (1.0 dex) to fit
the observations. Furthermore, these models imply a 7Li abundance about 0.2 dex
higher than observed for metal-poor disk stars.
