The pronoun object of a preposition is typically in the accusative form but may also be reflexive (e.g., She sent it to herself) or independent genitive (e.g., I hadn't heard of theirs.).
In a prepositional phrase, the object typically appears in the accusative case; however, it can also be reflexive (like "She sent it to herself") or an independent genitive (such as "I hadn't heard of theirs").