  In this paper we consider periodic orbits of planar linear Filippov systems
with a line of discontinuity. Unlike many publications researching only the
maximum number of crossing periodic orbits, we investigate not only the number
and configuration of sliding periodic orbits, but also the coexistence of
sliding periodic orbits and crossing ones. Firstly, we prove that the number of
sliding periodic orbits is at most 2, and give all possible configurations of
one or two sliding periodic orbits. Secondly, we prove that two sliding
periodic orbits coexist with at most one crossing periodic orbit, and one
sliding periodic orbit can coexist with two crossing ones.
