Oxygen-isotope effect on the in-plane penetration depth in underdoped   Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} as revealed by muon-spin rotation
  The oxygen-isotope (^16O/^18O) effect (OIE) on the in-plane penetration depth $\lambda_{ab} (0)$ in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} was studied by muon-spin rotation. A pronounced OIE on $\lambda_{ab}^{-2}(0)$ was observed with a relative isotope shift of $\Delta\lambda^{-2}_{ab}/\lambda^{-2}_{ab}$=-5(2)% for x =0.3 and -9(2)% for x=0.4. It arises mainly from the oxygen-mass dependence of the in-plane effective mass $m_{ab}^{\ast}$. The OIE exponents of T_{c} and of $\lambda_{ab}^{-2}(0)$ exhibit a relation that appears to be generic for cuprate superconductors. 
R.Khasanov, A.Shengelaya, K.Conder, E.Morenzoni, I.M.Savi\'c, and   H.Keller