  In this note, we prove the finite ${(Q-1)}$-Hausdorff measure of the free
boundary in the obstacle problem in a Carnot group $\mathbb{G}$. Here, $Q$
represents the homogeneous dimension of $\mathbb{G}$. Our main result, Theorem
1.1, constitutes the subelliptic counterpart of the Euclidean result due to
Caffarelli, but the analysis is complicated by the lack of commutation of the
left-invariant vector fields. This obstruction is compensated by the use of
right-invariant derivatives, and by a delicate compactness argument inspired to
Caffarelli's fundamental works.
