  In the scattering of positive pions by nuclei the double-charge-exchange
(DCX) reaction creates the possibility of formation of pionic atom states in
the vicinity of the threshold of this reaction. These quasi stationary states
can manifest themselves as resonances in the elastic scattering cross section.
The strength and shape of these resonances is strongly affected by an
instability of the nucleus created in the DCX channel. It is shown that this
mechanism can explain oscillation structures in the excitation function
observed in scattering of low-energy positive pions from $^{12}\text{C}$.
