Generalized orthotoric Kahler surfaces

Abstract

In this paper we describe QCH K\"ahler surfaces $(M,g,J)$ of generalized orthotoric type. We introduce a distinguished orthonormal frame on $(M,g)$ and give the structure equations for $(M,g,J)$. In the case when $I$ is conformally K\"ahler and $(M,g,J)$ is not hyperk\"ahler we integrate these structure equations and rediscover the orthotoric K\"ahler surfaces. We also investigate the hyperk\"aler case. We prove in a simple way that if $(M,g,J)$ is a hyperk\"ahler surface with a degenerate Weyl tensor $W^-$ (i.e. QCH hyperk\"ahler surface) then among all hyperk\"aler structures on $(M,g)$ there exists a K\"ahler structure $J_0$ such that $(M,g,J_0)$ is of Calabi type or of orthotoric type.