  High-resolution resonant inelastic x-ray scattering has been used to
determine the momentum dependence of orbital excitations in Mott-insulating
LaTiO$_3$ and YTiO$_3$ over a wide range of the Brillouin zone. The data are
compared to calculations in the framework of lattice-driven and
superexchange-driven orbital ordering models. A superexchange model in which
the experimentally observed modes are attributed to two-orbiton excitations
yields the best description of the data.
