  We observe that, for each positive integer n > 2, each of the Artin groups of
finite type A_n, B_n=C_n, and affine type \tilde A_{n-1} and \tilde C_{n-1} is
a central extension of a finite index subgroup of the mapping class group of
the (n+2)-punctured sphere. (The centre is trivial in the affine case and
infinite cyclic in the finite type cases). Using results of Ivanov and Korkmaz
on abstract commensurators of surface mapping class groups we are able to
determine the automorphism groups of each member of these four infinite
families of Artin groups.
