  We study the stability properties of orbits in dispersing billiards in a
homogeneous magnetic field by using a modified formalism based on the
Bunimovich-Sinai curvature (horocycle method). We identify simple periodic
orbits that can be stabilized by the magnetic field in the four-disk model and
the square-lattice Lorentz gas. The stable orbits can play a key role in
determining the transport properties of these models.
