  This paper concerns two-dimensional Filippov systems --- ordinary
differential equations that are discontinuous on one-dimensional switching
manifolds. In the situation that a stable focus transitions to an unstable
focus by colliding with a switching manifold as parameters are varied, a simple
sufficient condition for a unique local limit cycle to be created is
established. If this condition is violated, three nested limit cycles may be
created simultaneously. The result is achieved by constructing a Poincar\'e map
and generalising analytical arguments that have been employed for continuous
systems. Necessary and sufficient conditions for the existence of
pseudo-equilibria (equilibria of sliding motion on the switching manifold) are
also determined. For simplicity only piecewise-linear systems are considered.
