  There is a natural construction which associates to a finitely generated,
countable, discrete group $G$ and a 3-cocycle $\omega$ of $G$ an inclusion of
II$_1$ factors, the so-called diagonal subfactors (with cocycle). In the case
when the cocycle is trivial these subfactors are well studied and their
standard invariant (or planar algebra) is known. We give a description of the
planar algebra of these subfactors when a cocycle is present. The action of
Jones' planar operad involves the 3-cocycle $\omega$ explicitly and some
interesting identities for 3-cocycles appear when naturality of the action is
verified.
