Absence of the18anomaly in the oxygen isotope effect ofYBa2Cu3Oy

We have prepared oxygen isotope exchanged crystals of impurity-free YBCO with various oxygen concentents, and examined pure doping ($p$) dependance of isotope effect on superconducting transition temperature. With decreasing oxygen contents, the isotope exponent $\alpha$ monotonously increases without any anomaly around $p = 1/8$. The monotonous increase in $\alpha$ indicates that phonons are involved in the mechanism which causes the monotonous $T_c$ suppression with underdoping.