Oxygen isotope effect in the ab-plane reflectance of underdoped YBa(2)Cu(3)O(7-delta)

We have measured the effect of oxygen isotope substitution on the ab-plane reflectance of underdoped YBa(2)Cu(3)O(7-delta). The frequency shift of the transverse optic phonons due to the substitution of 16O by 18O yields an isotope effect of the expected magnitude for copper-oxygen stretching modes with alpha=0.5+/-0.1. The reflectance shoulder at 400-500 cm(-1) shows a much smaller exponent of alpha=0.1+/-0.1 in the normal state and alpha=0.23+/-0.1 in the superconducting state. These observations suggest that the shoulder is of electronic origin and not due to a phonon mode as has been suggested recently.