  General field theory of a fluctuating d-wave superconductor is constructed
and proposed as an effective description of superconducting cuprates at low
energies. The theory is used to resolve a puzzle posed by recent experiments on
superfluid density in severely underdoped YBCO. In particular, the overall
temperature dependence of the superfluid density at low dopings is argued to be
described well by the strongly anisotropic weakly interacting three-dimensional
Bose gas, and thus approximately linear in temperature with an almost
doping-independent slope.
