  The recently reported data on the yield ratio $\pi^-/\pi^+$ in central
rapidity region of heavy-ion collisions are analyzed by theoretical formula
which accounts for Coulomb interaction between central charged fragment (CCF)
consisting of nearly stopped nucleons with effective charge
$Z_{\mbox{\scriptsize eff}}$ and charged pions produced in the same region of
the phase space. The Coulomb wave function method is used instead of the usual
Gamow factor in order to account for the finite production range of pions,
$\beta$. For Gaussian shape of the pion production sources it results in a
quasi-scaling in $\beta$ and $Z_{\mbox{\scriptsize eff}}$ which makes
determination of parameters $\beta$ and $Z_{\mbox{\scriptsize eff}}$ from the
existing experimental data difficult. Only sufficiently accurate data taken in
the extreme small $m_{\scriptscriptstyle T}$-$m_{\pi}$ region, where this
quasi-scaling is broken, could be used for this purpose.
