The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters

High dispersion spectra (R greater than or similar to 40 000) for a quite l arge number of stars at the main sequence turn-off and at the base of the g iant branch in NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLT UT2). The [Fe/H] values we found are -2.03 +/- 0.02 +/- 0.04 and -1.42 +/- 0.02 +/- 0.04 for NGC 6397 and NGC 6752 respectively, where the first error bars refer to internal and the second ones to systematic errors (with in the abundance scale defined by our analysis of 25 subdwarfs with good Hi pparcos parallaxes). In both clusters the [Fe/H]'s obtained for TO-stars ag ree perfectly (within a few percent) with that obtained for stars at the ba se of the RGB. The [O/Fe] = 0.21 +/- 0.05 value Re obtain for NGC 6397 is q uite low, but it agrees with previous results obtained for giants in this c luster. 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 t he other hand, our results show clearly and for the first time that the O-N a anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular clus ter NGC 6752, a more massive cluster than NGC 6397. A similar anticorrelati on is present also for Mg and Al, and C and N. It is very difficult to expl ain the observed Na-OI and Mg-Al anticorrelation in NGC 6752 stars by a dee p mixing scenario; we think it requires some non internal mechanism.