  Slow oscillations (SlO) of the in-plane magnetoresistance with a frequency
less than 4 T are observed in the rare-earth tritellurides and proposed as an
effective tool to explore the electronic structure in various strongly
anisotropic quasi-two-dimensional compounds. Contrary to the usual
Shubnikov-de-Haas oscillations, SlO originate not from small Fermi-surface
pockets, but from the entanglement of close frequencies due to a finite
interlayer transfer integral, either between the two Te planes forming a
bilayer or between two adjacent bilayers. From the observed angular dependence
of the frequency and the phase of SlO we argue that they originate from the
bilayer splitting rather than from the Fermi-surface warping. The SlO frequency
gives the value of the interlayer transfer integral $\approx 1$ meV for
TbTe$_3$ and GdTe$_3$.
