  Observations of semi-detached interacting binaries reveal orbital modulation
indicating the presence of absorbing material obscuring the disk and accreting
primary star at specific orbital phases consistent with L1 stream material
overflowing the disk edge. We simulate the L1 stream interaction with the disk
using tests particles within the context of the Roche model in the restricted
three-body problem. At the disk bow shock the L1 stream particles are deflected
and launched onto ballistic trajectories above the disk (as would normally
occurs at the front of a detached shock in the hypersonic flow past a blunt
body).At a given scale height, the material is assumed to continue without
being affected by the disk, while at lower altitude it is being launched at an
increasing elevation, as well as gradually being dragged by the Keplerian flow.
We follow the stream material ballistic trajectories over the disk surface,
where they reach a maximum height z/r at a binary phase $\Phi \sim 0.75$, and
land onto the disk at a smaller radius around phase $\Phi \sim 0.5$. The radial
velocity for each $L1$ stream ballistic trajectory along the line of sight (of
the observer) to the hot inner parts of the disk is computed as a function of
the orbital phase for a binary configuration matching the dwarf nova U
Geminorum. The computed velocity amplitudes, phases, and pattern match the
observed velocity offsets of the metal lines in the FUSE spectrum of U Gem
during outburst. As ballistic trajectories are much easier to compute than
realistic three-dimensional hydrodynamical simulations, we propose the use of
the L1 stream deflection and ballistic launching as a means for the analysis of
the absorption lines orbital variability in semi-detached binaries and to
assess or confirm, with some limitations, the system parameters such as the
mass ratio, inclination, and disk outer radius.
