Invariant embeddings and weighted permutations

We prove that for any fixed unitary matrix $U$, any abelian self-adjoint algebra of matrices that is invariant under conjugation by $U$ can be embedded into a maximal abelian self-adjoint algebra that is still invariant under conjugation by $U$. We use this result to analyse the structure of matrices $A$ for which $A^{\ast }A$ commutes with $A{A}^{\ast }$, and to characterize matrices that are unitarily equivalent to weighted permutations.