  For an epimorphism pi of the free group F_n onto a finite group G write
Gamma(G,pi) for the group of all automorphisms f of F_n for which pi*f = pi.
This is called the standard congruence subgroup of Aut(F_n) associated to G and
pi. In the case n = 2 we present formulas for the index of Gamma(G,pi) where G
is abelian or dihedral. Moreover, we show that congruence subgroups associated
to dihedral groups provide a family of subgroups of arbitrary large index in
Aut(F_2) generated by a fixed number of elements. This implies that finite
index subgroups of Aut(F_2) cannot be written as free products.
