What information can we obtain from the yield ratio $\pi^-/\pi^+$ in   heavy-ion collisions ?
  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. 
T. Osada, S.Sano, M.Biyajima and G.Wilk