Self-dual almost-Kähler four manifolds

We classify compact self-dual almost-Kähler four manifolds of positive type and zero type. In particular, using LeBrun’s result, we show that any self-dual almost-Kähler metric on a manifold which is diffeomorphic to ${\mathbb{C}\mathbb{P}}_2$ is the Fubini-Study metric up to rescaling. In case of negative type, we classify compact self-dual almost-Kähler four manifolds with $J$-invariant Ricci tensor.