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Deep WFPC2 images in wide bands centered at 606 and 802 nm were taken with the HST 5.6 arcminutes away from the centers of the galactic globular clusters NGC~6397 and M15. The images were used to accurately position respectively $\sim 2120$ and $\sim 7780$ stars detected in the fields in two color-magnitude diagrams extending down to a limiting magnitude $m_{814} \simeq m_I \simeq 26$ determined reliably and solely by counting statistics. A white dwarf sequence and a rich, narrow cluster main sequence (MS) are detected in both clusters, the latter stretching from $m_{814}=18.5$ to $m_{814} \simeq 25$. At $m_{814} \simeq 24$ the MS of NGC~6397 becomes indistinguishable from the field population. The corresponding luminosity functions increase slowly from $M_{814} \simeq 6.5$ to 8.5 as expected from ground based observations but then drop sharply from there down to the measurement limit. The corresponding mass functions (MF) obtained by using the only presently available mass-luminosity relation for the clusters' metallicity rise to a plateau between $\sim 0.25$ and$\sim 0.15$ M$_\odot$ (the measurement limit for M15), and then the MF of NGC~6397 clearly drops towards the expected mass limit of the normal hydrogen burning main sequence at about $0.1$ M$_\odot$. This result is in clear contrast to that obtained from the ground and implies either a substantial modification of the clusters' IMFs due to dynamical evolution and/or interaction with the Galaxy in their lifetime (such as disk or bulge shocking, which is plausible for NGC~6397, but very unlikely for M15, given its scale height), or that very low mass stars are not produced in any dynamically significant amount by clusters of this type. For both clusters the white dwarf sequence is in reasonable agreement with a cooling sequence of C-O models of mass $0.5$ M$_\odot$ at the canonical distance of the two objects, with a scatter that is most likely due to photometric errors, but may also reflect real differences in mass or chemical composition. Contamination from unresolved galaxies, which can not be reliably identified with our filters, makes it difficult to meaningfully compare the observed white dwarf luminosity functions with their theoretical counterparts.
