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We present (V,V-I) VLT-FORS1 observations of the Galactic Globular ClusterNGC 6397. We derive accurate color--magnitude diagrams and luminosity functions(LFs) of the cluster Main Sequence (MS) for two fields extending from a regionnear the centre of the cluster out to ~ 10 arcmin. The photometry of thesefields produces a narrow MS extending down to V ~ 27, much deeper than anyprevious ground based study on this system and comparable to previous HSTphotometry. The V, V-I CMD also shows a deep white dwarf cooling sequencelocus, contaminated by many field stars and spurious objects. We concentratethe present work on the analysis of the MSLFs derived for two annuli atdifferent radial distance from the center of the cluster. Evidence of aclear-cut correlation between the slope of the observed LFs before reaching theturn-over, and the radial position of the observed fields inside the clusterarea is found. We find that the LFs become flatter with decreasing radius (x ~0.15 for 1'< R1 < 5.5'; x ~ 0.24 for 5.5'< R2 <9.8'; core radius, rc = 0.05'),a trend that is consistent with the interpretation of NGC 6397 as a dynamicallyrelaxed system. This trend is also evident in the mass function.Comment: 11 pages; 15 figures; accepted for publication on Astronomy and  Astrophysic

VLT FORS-1 observations of NGC 6397: Evidence for mass segregation