Supersymmetric Black Holes

The effective action of $N=2$, $d=4$ supergravity is shown to acquire no quantum corrections in background metrics admitting super-covariantly constant spinors. In particular, these metrics include the Robinson-Bertotti metric (product of two 2-dimensional spaces of constant curvature) with all 8 supersymmetries unbroken. Another example is a set of arbitrary number of extreme Reissner-Nordstr\"om black holes. These black holes break 4 of 8 supersymmetries, leaving the other 4 unbroken. We have found manifestly supersymmetric black holes, which are non-trivial solutions of the flatness condition $\cd^{2} = 0$ of the corresponding (shortened) superspace. Their bosonic part describes a set of extreme Reissner-Nordstr\"om black holes. The super black hole solutions are exact even when all quantum supergravity corrections are taken into account.