Last year Pablo Fillottrani and I introduced an ontology-driven unifying metamodel of the static, structural, entities of UML Class Diagrams (v2.4.1), ER, EER, ORM, and ORM2 in [1,2], which was informally motivated and described here. This now also includes the constraints and we have formalised it in First Order Predicate Logic to put some precision to the UML Class Diagram fragments and their associated textual constraints, which is described in the technical report of the metamodel formalization . Besides having such precision for the sake of it, it is also useful for automated checking of inter-model assertions and computing model transformations, which we illustrated in our RuleML’14 paper earlier this year  (related blog post).
The ‘bummer’ of the formalization is that it probably requires an undecidable language, due to having formulae with five variables, counting quantifiers, and ternary predicates (see section 2.11 of the tech report for details). To facilitate various possible uses nevertheless, we therefore also made a slightly simpler OWL version of it (the modelling decisions are described in Section 3 of the technical report). Having that OWL version, it was easy to also generate a verbalisation of the OWL version of the metamodel (thanks to SWAT NL Tools) so as to facilitate reading of the ontology by the casually interested reader and the very interested one who doesn’t like logic.
Although our DST/MINCyT-funded South Africa-Argentina scientific collaboration project (entitled Ontology-driven unification of conceptual data modelling languages) is officially in its last few months by now, more results are in the pipeline, which I hope to report on shortly.
 Keet, C.M., Fillottrani, P.R. Toward an ontology-driven unifying metamodel for UML Class Diagrams, EER, and ORM2. 32nd International Conference on Conceptual Modeling (ER’13). 11-13 November, 2013, Hong Kong. Springer LNCS vol 8217, 313-326.
 Keet, C.M., Fillottrani, P.R. Structural entities of an ontology-driven unifying metamodel for UML, EER, and ORM2. 3rd International Conference on Model & Data Engineering (MEDI’13). September 25-27, 2013, Amantea, Calabria, Italy. Springer LNCS (in print).
 Fillottrani, P.R., Keet, C.M. Conceptual Model Interoperability: a Metamodel-driven Approach. 8th International Web Rule Symposium (RuleML’14), A. Bikakis et al. (Eds.). Springer LNCS 8620, 52-66. August 18-20, 2014, Prague, Czech Republic.