The rigidity problem for analytic critical circle maps
  It is shown that if $f$ and $g$ are any two analytic critical circle mappings with the same irrational rotation number, then the conjugacy that maps the critical point of $f$ to that of $g$ has regularity $C^{1+\alpha}$ at the critical point, with a universal value of $\alpha>0$. As a consequence, a new proof of the hyperbolicity of the full renormalization horseshoe of critical circle maps is given. 
D. Khmelev and M. Yampolsky