  Imaging atmospheric Cherenkov telescopes (IACTs) such as the Major
Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes endeavor to reach
the lowest possible energy threshold. In doing so the trigger system is a key
element. Reducing the trigger threshold is hampered by the rapid increase of
accidental triggers generated by ambient light, the so-called Night Sky
Background (NSB). In this paper we present a topological trigger, dubbed
Topo-trigger, which rejects events on the basis of their relative orientation
in the telescope cameras. We have simulated and tested the trigger selection
algorithm in the MAGIC telescopes. The algorithm was tested using MonteCarlo
simulations and shows a rejection of 85% of the accidental stereo triggers
while preserving 99 % of the gamma rays. A full implementation of this trigger
system would achieve an increase in collection area between 10 and 20% at the
energy threshold. The analysis energy threshold of the instrument is expected
to decrease by ~8 %. The selection algorithm was tested on real MAGIC data
taken with the current trigger configuration and no $\gamma$-like events were
found to be lost.
