  Electron-positron momentum densities in Y, reconstructed from two-dimensional
angular correlation of annihilation radiation spectra, are compared with the
theoretical predictions of fully-relativistic augmented plane-wave
calculations. Knowledge of the theoretical densities and of the effects on them
of certain symmetry selection rules has allowed us to separate two hole Fermi
sur-faces in the third and fourth bands and to establish some Fermi momenta for
each of them.
