Consistent gravitational anomalies for chiral scalars

Starting from the Henneaux-Teitelboim action for a chiral scalar, which generalizes to curved space the Floreanini-Jackiw action, we give two simple derivations of the exact consistent gravitational anomaly. The first derivation is through the Schwinger-DeWitt regularization. The second exploits cohomological methods and uses the fact that in dimension two the diffeomorphism transformations are described by a single ghost which allows to climb the cohomological chain in a unique way.