A characterization of relative pure injective R-modules

Let R be a commutative ring with identity and S be any class of finitely presented R-modules containing R. Then the class of all S-pure injective R-modules is enveloping. Moreover, an Rmodule M is S-pure injective if and only if it is isomorphic to a direct summand of an R-module of the form $\prod_{i\in I}U_{i}^{+}$ where Ui is isomorphic to an R-module in add(tr(S)) for all $i\in I$