A Kummer type construction of self-dual metrics on the connected sum of four complex projective planes

We show that there exist on 4CP(2), the connected sum of four complex projective planes, self-dual metrics with the following properties: (i) the sign of the scalar curvature is positive, (ii) the identity component of the isometry group is U(1), (iii) the metrics are not conformally isometric to the self-dual metrics constructed by LeBrun [LB1]. These are the first examples of self-dual metrics with cion semi-free U(1)-isometries on simply connected manifolds. Our proof is based on the twister theory: we use an equivariant orbifold version of the construction of Donaldson and Friedman [DF]. We also give a rough description of the structure of the algebraic reduction of the corresponding twister spaces.