Fermi surface arcs and the infrared conductivity of underdoped YBa$_{2}$Cu$_{3}$O$_{6.5}$

Using recent finding, that the electronic states lost below the pseudogap energy ($\Delta _{pg})$ are recovered in the energy region immediately above it, we analyze the in-plane far infrared conductivity data in underdoped orthoII YBa$_{2}$Cu$_{3}$O$_{6.5}$ and are able to find evidence for the opening of a pseudogap on part of the Fermi surface with the remaining ungaped piece proportional to the temperature. These results are similar to recent angle-resolved photoemission spectroscopy data in underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$.