A logarithm law for automorphism groups of trees

Let \Gamma be a geometrically finite tree lattice. We prove a Khintchine-Sullivan type theorem for the Hausdorff measure of the points at infinity of the tree that are well approximated by the parabolic fixed points of G. Using Bruhat-Tits trees, an application is given for the Diophantine approximation of formal Laurent series in the variable 1/X over the finite field Fq by rational fractions in X over Fq, satisfying some congruence properties