BPS states and supersymmetry

We describe duality invariant constraints, which allow a classification of BPS states preserving different fractions of supersymmetry. We then relate this analysis to the orbits of the exceptional groups E/sub 6(6)/, E/sub 7(7)/, relevant for black holes in five and four dimensions. The orbits of O(P, Q) groups enter in the classification of the BPS string in six dimensions. (25 refs).