  We consider a compact metric graph of size $\varepsilon$, and attach to it
several edges (leads) of length of order one (or of infinite length). As
$\varepsilon$ goes to zero, the graph $\mathcal{G}^\varepsilon$ obtained in
this way looks like the star-graph formed by the leads joined in a central
vertex. On $\mathcal{G}^\varepsilon$ we define an Hamiltonian $H^\varepsilon$,
properly scaled with the parameter $\varepsilon$. We prove that there exists a
scale invariant effective Hamiltonian on the star-graph that approximates
$H^\varepsilon$ (in a suitable norm resolvent sense) as $\varepsilon\to0$. The
effective Hamiltonian depends on the spectral properties of an auxiliary
$\varepsilon$-independent Hamiltonian defined on the compact graph obtained by
setting $\varepsilon = 1$. If zero is not an eigenvalue of the auxiliary
Hamiltonian, in the limit $\varepsilon\to0$, the leads are decoupled.
