  We have determined Li abundances for 11 G-type stars in the 6-8 Gyr old open
cluster NGC 188. These data significantly enlarge the number of cluster stars
with Li measurements, allowing us to extend the investigation of Li depletion
in open clusters to ages well beyond the age of the Sun. We have also inferred
the cluster metallicity which turns out to be solar. We find that solar-type
stars in NGC 188 are only slightly more Li depleted than the much younger
Hyades and no more Li depleted than stars of similar temperature in the 2-4 Gyr
younger cluster M 67. At variance with M 67, NGC 188 members show virtually no
scatter in their Li abundances. Surprisingly, no solar- type star in NGC 188
appears as Li depleted as the Sun or as the most Li depleted stars in M 67. We
discuss the implications of these results for mechanisms of internal mixing and
Li depletion in main sequence stars.
