High dispersion spectra ( R \lower 2.58 pt \hbox { $ \sim$ } \kern - 7.75 pt \raise 2.795 pt \hbox { $ > $ } 40 , 000 ) for a quite large number of stars at the main sequence turn-off and at the base of the subgiant branch in NGC6397 and NGC6752 were obtained with the UVES on Kueyen ( VLT UT2 ) .
The [ Fe/H ] values we found are -2.03 \pm 0.02 \pm 0.04 and -1.42 \pm 0.02 \pm 0.04 for NGC6397 and NGC6752 respectively , where the first error bars refer to internal and the second ones to systematic errors .
In both clusters the [ Fe/H ] ’ s obtained for TO-stars agree perfectly ( within a few per cents ) with that obtained for stars at the base of the RGB .
The [ O/Fe ] = 0.21 \pm 0.05 value we obtain for NGC6397 is quite low , but it conforms to previous results obtained for giants in this cluster .
Moreover , the star-to-star scatter in both O and Fe is very small , indicating that this small mass cluster is chemically very homogenous .
On the other side , our results show clearly and for the first time that the O-Na anticorrelation ( up to now seen only for stars on the red giant branches of globular clusters ) is present among unevolved stars in the globular cluster NGC6752 , a more massive cluster than NGC6397 .
A similar anticorrelation is present also for Mg and Al , and C and N. It is very difficult to explain the observed Na-O , and Mg-Al anticorrelation in NGC6752 stars by a deep mixing scenario ; we think it requires some non internal mechanism .