SUPERGEOMETRY OF 3-DIMENSIONAL BLACK-HOLES

We show how the supersymmetric properties of three-dimensional black h oles can be obtained algebraically. The black hole solutions are const ructed as quotients of the supergroup OSp(1\2;R) by a discrete subgrou p of its isometry supergroup. The generators of the action of the isom etry supergroup which commute with these identifications are found. Th ese yield the supersymmetries for the black hole as found in recent st udies as well as the usual geometric isometries. It is also shown that , in the limit of a vanishing cosmological constant, the black hole va cuum becomes a null orbifold, a solution previously discussed in the c ontext of string theory.