  We study the oxygen isotope (^{16}O,^{18}O) and finite size effects in
Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7-\delta} by in-plane penetration depth (\lambda
_{ab}) measurements. A significant change of the length L_{c} of the
superconducting domains along the c-axis and \lambda_{ab}^{2} is deduced,
yielding the relative isotope shift \Delta L_{c}/L_{c}\approx \Delta \lambda
_{ab}^{2}/\lambda_{ab}^{2}\approx -0.14 for x=0, 0.2 and 0.3. This uncovers the
existence and relevance of the coupling between the superfluid, lattice
distortions and anharmonic phonons which involve the oxygen lattice degrees of
freedom.
