Let \(C_\varphi \) be a composition operator acting on the Hardy space of the unit disc \(H^p\) ( \(1\le p < \infty \) ), which is embedded in a \(C_0\) -semigroup of composition operators \(\mathcal {T}=(C_{\varphi _t})_{t\ge 0}.\) We investigate whether the commutant or the bicommutant of \(C_\varphi \) , or the commutant of the semigroup \(\mathcal {T}\) , are isomorphic to subalgebras of continuous functions defined on a connected set. In particular, it allows us to derive results about the existence of non-trivial idempotents (and non-trivial orthogonal projections if \(p=2\) ) lying in such sets. Our methods also provide results concerning the minimality of the commutant and the double commutant property, in the sense that they coincide with the closure in the weak operator topology of the unital algebra generated by the operator. Moreover, some consequences regarding the extended eigenvalues and the strong compactness of such operators are derived. This extends previous results of Lacruz, León-Saavedra, Petrovic and Rodríguez-Piazza, Fernández-Valles and Lacruz and Shapiro on linear fractional composition operators acting on \(H^2\) .
