  Let $G$ be a group. The central automorphism group $Aut_c(G)$ of $G$ is the
centralizer of $Inn(G)$ the subgroup of $Aut(G)$ of inner automorphisms. There
is a one to one map $ \sigma \mapsto h_\sigma$ from the set $Aut_c(G)$ onto the
set $Hom(G,Z(G))$ of homomorphisms from $G$ onto its center, with $
h_\sigma(x)=x^{-1} \sigma(x)$. This map can be used to obtain informations
about the size of $Aut_c(G)$, and also about its structure in some special
cases. In this paper we see how to use it to obtain informations about the
structure of $Aut_c(G)$ in the general case. The notion of the adjoint group of
a ring is the main tool in our approach.
