  Topological euclidean gravity is built in eight dimensions for manifolds with
$Spin(7) \subset SO(8)$ holonomy. In a previous work, we considered the
construction of an eight-dimensional topological theory describing the graviton
and one graviphoton. Here we solve the question of determining a topological
model for the combined system of a metric and a Kalb--Ramond two-form gauge
field. We then recover the complete $N=1, D=8$ supergravity theory in a twisted
form. We observe that the generalized self-duality conditions of our model
correspond to the octonionic string equations.
