  We refine Cohen and Lustig's description of centralisers of Dehn twists of
free groups. We show that the centraliser of a Dehn twist of a free group has a
subgroup of finite index that has a finite classifying space. We describe an
algorithm to find a presentation of the centraliser. We use this algorithm to
give an explicit presentation for the centraliser of a Nielsen automorphism in
Aut(F_n). This gives restrictions to actions of Aut(F_n) on CAT(0) spaces.
