  Slow oscillations of the interlayer magnetoresistance observed in the layered
organic metal $\beta $-(BEDT-TTF)$_2$IBr$_2$ are shown to originate from the
slight warping of its Fermi surface rather than from independent small
cyclotron orbits. Unlike the usual Shubnikov-de Haas effect, these oscillations
are not affected by the temperature smearing of the Fermi distribution and can
therefore become dominant at high enough temperatures. We suggest that the slow
oscillations are a general feature of clean quasi-two-dimensional metals and
discuss possible applications of the phenomenon.
