  EPW-sextics are special 4-dimensional sextic hypersurfaces (with 20 moduli)
which come equipped with a double cover. We analyze the double cover of
EPW-sextics parametrized by a certain prime divisor in the moduli space. We
associate to the generic sextic parametrized by that divisor a K3 surface of
genus 6 and we show that the double epw sextic is a contraction of the Hilbert
square of the K3. This result has two consequences. First it gives a new proof
of the following result of ours: smooth double EPW-sextics form a locally
complete family of hyperkaehler projective deformations of the Hilbert square
of a K3. Secondly it shows that away from another prime divisor in the moduli
space the period map for double EPW-sextics is as well-behaved as it possibly
could be.
