Symmetries of K3 sigma models

It is shown that the supersymmetry-preserving automorphisms of any non-linear σ-model on K3 generate a subgroup of the Conway group Co1. This is the stringy generalisation of the classical theorem, due to Mukai and Kondo, showing that the symplectic automorphisms of any K3 manifold form a subgroup of the Mathieu group M23. The Conway group Co1 contains the Mathieu group M24 (and therefore in particular M23) as a subgroup. We confirm the predictions of the Theorem with three explicit CFT realisations of K3: the T 4 /Z2 orbifold at the self-dual point, and the two Gepner models (2) 4 and (1) 6 . In each case we demonstrate that their symmetries do not form a subgroup of M24, but lie inside Co1 as predicted by our