Minimal Higgs branch for the breaking of half of the supersymmetries in N = 2 supergravity

It is shown that the minimal Higgs sector of a generic N = 2 supergravity theory with unbroken N = 1 supersymmetry must contain a Higgs hypermultiplet and a vector multiplet. When the multiplets parametrize the quaternionic manifold SO(4,1)SO(4), and the special Kähler manifold SU(1,1)U(1), respectively, a vanishing vacuum energy with a sliding massive spin 32 multiplet is obtained. Potential applications to N = 2 low energy effective actions of superstrings are briefly discussed.