  The Ortho-II phase of YBa$_2$Cu$_3$O$_6.5$ is characterized by a periodic
alternation of empty Cu and filled Cu-O b-axis doubling the unit cell in the a
direction. The extra oxygen in the full chains gives rise to an attractive
potential for the holes in the planes. The planar bands split in two with a gap
opening at the new Brillouin zone boundary $k_x=\pm pi/2$, which we estimate
from LDA calculations. Using a planar model which treats the d-wave
superconductivity in a mean field approximation, we show that interband
transitions produce a strongly anisotropic feature in the optical conductivity
controlled by a region in k-space close to ($\pi/2,\pi/2$). The edge position
of this feature gives information on the temperature dependence of
quasiparticle spectrum in this region. Bilayer splitting would show up as a
double edge shape.
