  Spin gap in optimally-doped YBCO was investigated by inelastic neutron
scattering. We found that q-integrated spectral weight in the vicinity of the
spin gap has a shape of a step function at 10K with sharp resolution-limited
onset at 33 meV, i.e. there is no observable spectral weight at 31 meV but full
spectral weight at 35 meV. The spin gap does not soften or "fill in" as the
temperature is increased to Tc. Above Tc we find no evidence for q-dependent
magnetic signal at 35 meV.
