  Starting from the Kubo formula for conductance, we calculate the
frequency-dependent response of a single-electron transistor (SET) driven by an
ac signal. Treating tunneling processes within the lowest order approximation,
valid for a wide range of parameters, we discover a finite reactive part even
under Coulomb blockade due to virtual processes. At low frequencies this can be
described by an effective capacitance. This effect can be probed with microwave
reflection measurements in radio-frequency (rf) SET provided that the
capacitance of the surroundings does not completely mask that of the SET.
