“…Furthermore, in dimension 4, any hyper-symplectic structure underlies an anti-self-dual (ASD) and Ricci-flat neutral metric. For this reason such structures have been used in string theory [42,30,33,3,31,10,26] and integrable systems [16,4,17]. In fact, N = 2 superstring theory is considered in [42], and it is proved that the critical dimension of such a string is 4 and that the bosonic part of the N = 2 theory corresponds to self-dual (SD) metrics of signature (2,2).…”