  The form of resonance line-shapes unveils information about its
nonperturbative properties and formation mechanisms. Here, we study the
non-Breit-Wigner energy distribution of the resonance $\psi (3770)$ using an
unitarized effective Lagrangian approach, that includes the effect of the
nearby threshold $D^{+}D^{-}$. Two poles are found in the second Riemann sheet
near the resonance amplitude. We discuss the setting of the free parameters and
possible effects contributing to the signal.
