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We report sound-velocity measurements on polycrystalline single-phase Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ with a ${T}_{c}$ of 90 K. The sound velocity is found to increase as the temperature is reduced from room temperature in contrast to the decrease observed in ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_{4}$. At ${T}_{c}$ there is no discontinuity in the sound velocity within experimental resolution, as expected from thermodynamic arguments and the small value of $\frac{d{T}_{c}}{\mathrm{dP}}$. As the temperature is lowered below ${T}_{c}$ the sound velocity increases at a greater rate. A similar increase below ${T}_{c}$ is observed in ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_{4}$. The magnitude of the sound-velocity increase below ${T}_{c}$ is much larger than observed in conventional superconductors.

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