  In classical superconductors Cooper-pair formation and phase coherence occur
simultaneously as the temperature is lowered below Tc. In high-temperature
superconductors (HTSC), on the other hand, the small superfluid density and low
associated phase stiffness of the superconducting condensate are expected to
lead to a separation of the Cooper-pair formation and the phase-coherence
temperatures, especially in underdoped materials [3]. The only real phase
transition in this scenario is the 3d-XY phase-ordering transition at Tc [3,4].
In our Letter [2] we showed that Tc in underdoped and optimally doped YBCO is
just such a phase-ordering temperature, and then the question naturally arises
- where do the Cooper pairs form? The observed 3d-XY scaling of our thermal
expansion data over a wide temperature range [2] suggests that pairing occurs
at temperatures considerably above Tc, and it thus appeared quite natural for
us to associate the opening of the pseudogap at T*.
