High resolution ( R \simeq 110 000 ) , very high S/N spectra centered on the 6707.8 Å Li i line have been obtained with the ESO Coudé Echelle Spectrometer for five , metal-poor ( -0.8 < \mbox { [ 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 Li i line as a function of the lithium isotope ratio .
This has led to a detection of \@element [ ] [ 6 ] [ ] [ ] { \mathrm { Li } } in HD 68284 and HD 130551 with \@element [ ] [ 6 ] [ ] [ ] { \mathrm { Li } } / \@element [ ] [ 7 ] [ ] [ ] { \mathrm { Li } } \simeq 0.05 , whereas the other stars , HR 2883 , HR 3578 and HR 8181 , have \@element [ ] [ 6 ] [ ] [ ] { \mathrm { Li } } / \@element [ ] [ 7 ] [ ] [ ] { \mathrm { Li } } close to zero .

By comparing T _ { eff } -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 \@element [ ] [ 6 ] [ ] [ ] { \mathrm { Li } } present have a significantly higher mass , { \cal M } / { \cal M } _ { \sun } \simeq 1.05 , than the other three stars for which values between 0.85 and 1.0 { \cal 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 \@element [ ] [ 6 ] [ ] [ ] { \mathrm { Li } } abundances are in agreement with standard cosmic ray production of \@element [ ] [ 6 ] [ ] [ ] { \mathrm { Li } } in the galactic disk and a moderate depletion ( 0.5 dex ) in the stars .
Recent models for the evolution of \@element [ ] [ 6 ] [ ] [ ] { \mathrm { Li } } including \alpha + \alpha fusion reactions and predicting a high lithium isotopic ratio , \@element [ ] [ 6 ] [ ] [ ] { \mathrm { Li } } / \@element [ ] [ 7 ] [ ] [ ] { \mathrm { Li } } \simeq 0.3 at \mbox { [ Fe / H ] } = -0.6 , require a high degree of \@element [ ] [ 6 ] [ ] [ ] { \mathrm { Li } } depletion ( \simeq 1.0 dex ) to fit the observations .
Furthermore , these models imply a \@element [ ] [ 7 ] [ ] [ ] { \mathrm { Li } } abundance about 0.2 dex higher than observed for metal-poor disk stars .