  Slow oscillations of the in-plane magnetoresistance are observed in the
rare-earth tritellurides and proposed as an effective tool to determine the
parameters of electronic structure in various strongly anisotropic
quasi-two-dimensional compounds. These oscillations do not originate from the
small Fermi surface pockets, as revealed usually by the Shubnikov-de-Haas
oscillations, but from the entanglement of close frequencies due to a finite
interlayer transfer integral $t_z$, which allows to estimate its value. For
TbTe$_3$ and GdTe$_3$ we obtain the estimate $t_z\approx 1$ meV.
