Observation of an oxygen isotope effect in superconducting (${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The oxygen isotope effect was studied in the system (${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Pr}}_{\mathit{x}}$)${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ for x=0.2, 0.3, 0.4, and 0.5. It is found that the oxygen isotope exponent α increases with increasing x and therefore decreasing transition temperature ${\mathit{T}}_{\mathit{c}}$. For the highest Pr concentrations α tends toward α=0.5. The value of the isotope exponent depends on the concentration of mobile holes. The lower this concentration is, the larger becomes the isotope exponent.