Observations of semi-detached interacting binaries reveal orbital modulation in the optical , UV , and X-ray bands , indicating the presence of absorbing material obscuring the disk and accreting primary star at specific orbital phases consistent with L 1 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 a 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 . Near the disk mid-plane ( z < < H ) the material is assumed to become part of the disk . 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 phase of the maximum height , phase of the landing site and phase of the hot spot itself , all decrease significantly with decreasing disk radius . The radial velocity for each L 1 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 L 1 stream deflection and ballistic launching as a means for the analysis of the absorption-line orbital variability in semi-detached binaries and to assess or confirm , with some limitations , system parameters such as the mass ratio , inclination , and disk outer radius .