  Let $G$ be a residually finite group and let $A$ be a finite set. We prove
that if $X \subset A^G$ is a strongly irreducible subshift of finite type
containing a periodic configuration then periodic configurations are dense in
$X$. The density of periodic configurations implies in particular that every
injective endomorphism of $X$ is surjective and that the group of automorphisms
of $X$ is residually finite. We also introduce a class of subshifts $X \subset
A^\Z$, including all strongly irreducible subshifts and all irreducible sofic
subshifts, in which periodic configurations are dense.
