We first prove an isomorphism between the moduli space of smooth cubic threefolds and the moduli space of hyperkähler fourfolds of K3 [2] -type with a non-symplectic automorphism of order three, whose invariant lattice has rank one and is generated by a class of square 6; both these spaces are uniformized by the same 10-dimensional arithmetic complex ball quotient. We then study the degeneration of the automorphism along the loci of nodal or chordal degenerations of the cubic threefold, showing the birationality of these loci with some moduli spaces of hyperkähler fourfolds of K3 [2] -type with nonsymplectic automorphism of order three belonging to different families. Finally, we construct a cyclic Pfaffian cubic fourfold to give an explicit construction of a non-natural automorphism of order three on the Hilbert square of a K3 surface.Finally, in Section 5, looking at Hassett divisors on the moduli space of cubic fourfolds, we produce a Pfaffian cyclic cubic fourfold, thus we show: Corollary 1.3. There exists a smooth complex K3 surface whose Hilbert square admits a non-natural non-symplectic automorphism of order three, with fixed locus isomorphic to the Fano surface of a cubic threefold and invariant lattice isometric to 6 .We further explicitly describe this automorphism in terms of the Pfaffian geometry of the K3 surface, in analogy with Beauville's construction of the non-natural non-symplectic involution on the Hilbert scheme of a general quartic surface [10].Acknowledgements. The second author was partially supported by FIRB 2012 "Moduli spaces and their applications" and by "Laboratoire Internationale LIA LYSM". The authors thank Klaus Hulek, Shigeyuki Kondō, Christian Lehn, Gregory Sankaran and Davide Veniani for their helpful comments and suggestions.
Fano varieties of lines of cyclic cubic fourfoldsWe work over the field of complex numbers.