LAUSR

We present a free field realisation for the vertex operator algebra associated to the genus-two, class S superconformal field theory of type a1. The free field realisation is in the style of recent work by the authors, and is formulated in terms of a one-dimensional isotropic lattice vertex algebra along with two pairs of symplectic fermions. Our realisation makes manifest an enhanced USp(4) outer automorphism group of the VOA that is inherited from the symplectic fermion system. This extends an SU(2) outer automorphism that has been observed in recent work of Kiyoshige and Nishinaka and significantly simplifies the structure of the algebra. Along the way, we also produce a realisation of the generic subregular Drinfel’d–Sokolov W algebra of type c2 in terms of the generic principle W algebra of type c2 and a one-dimensional isotropic lattice vertex algebra.