On the group of a rational maximal bifix code

We give necessary and sufficient conditions for the group of a rational maximal bifix code $Z$ to be isomorphic with the $F$-group of $Zcap F$, when $F$ is recurrent and $Zcap F$ is rational. The case where $F$ is uniformly recurrent, which is known to imply the finiteness of $Zcap F$, receives special attention. The proofs are done by exploring the connections with the structure of the free profinite monoid over the alphabet of $F$.