  In the cuprate superconductor $YBa_2Cu_3O_{6+x}$, hole doping in the $CuO_2$
layers is controlled by both oxygen content and the degree of oxygen-ordering.
At the composition $\rm YBa_2Cu_3O_{6.35}$, the ordering can occur at room
temperature, thereby tuning the hole doping so that the superconducting
critical temperature gradually rises from zero to 20 K. Here we exploit this to
study the c-axis penetration depth as a function of temperature and doping. The
temperature dependence shows the d-wave superconductor surviving to very low
doping, with no sign of another ordered phase interfering with the nodal
quasiparticles. The only apparent doping dependence is a smooth decline of
superfluid density as Tc decreases.
