  Average mobility of very feebly damped particles in tilted periodic
potentials is considered. Under the combined action of thermal fluctuations and
small temporal modulation of the tilt of the potential the particles, in the
small tilt range, become more mobile than without modulation. The enhancement
of mobility depends (nonmonotonically) on the frequency of modulation. For
small modulations the enhancement shows a peak as a function of frequency. This
has an obvious implication on the measured voltage across a Josephson junction
driven by a small amplitude alternating current of suitable frequency.
