Full Idea
Theoretical identities, according to the conception I advocate, are generally identities involving rigid designators and therefore are examples of the necessary a posteriori.
Clarification
'Rigid designators' are baptismal names
Gist of Idea
Theoretical identities are between rigid designators, and so are necessary a posteriori
Source
Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 3)
Book Reference
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.140
A Reaction
This doesn't open up a huge new realm of a posteriori necessity. We just cured some of our ignorance. I remain unconvinced that the Morning Star is necessarily the Evening Star, except in the boring way that if it is, it is. Venus could fission.