Hilbert $C^*$-modules over $\Sigma^*$-algebras II: $\Sigma^*$-Morita equivalence

In previous work, we defined and studied $\Sigma^*$-modules, a class of Hilbert $C^*$-modules over $\Sigma^*$-algebras (the latter are $C^*$-algebras that are sequentially closed in the weak operator topology). The present work continues this study by developing the appropriate $\Sigma^*$-algebraic analogue of the notion of strong Morita equivalence for $C^*$-algebras. We define strong $\Sigma^*$-Morita equivalence, prove a few characterizations, look at the relationship with equivalence of categories of a certain type of Hilbert space representation, study $\Sigma^*$-versions of the interior and exterior tensor products, and prove a $\Sigma^*$-version of the Brown-Green-Rieffel stable isomorphism theorem.