First, we discuss the dynamics of a weakly charged dust grain orbiting in the equatorial plane of a planet surrounded by a rigidly corotating magnetosphere, where the magnetic field can be represented by a centered aligned dipole field. If the grain has a constant charge, we can introduce an effective one-dimensional potential, from which we find that the perturbation due to electrostatic forces induces a motion of the pericenter, similar to the effect of the planetary oblateness. However, the electric charge on an orbiting dust grain is generally not constant. Accordingly, we explore the case where the charge varies periodically due to the modulation of the photoelectron current that occurs as the grain enters and leaves the planetary shadow. Thus the electromagnetic perturbation resonates with the orbital period and can modify the orbit's size and eccentricity. We demonstrate this effect both numerically and analytically for small grains comprising the Jovian ring. We show that the resulting changes are periodic, and their amplitude is much bigger than the amplitude of the periodic changes due to light-pressure perturbation or the secular changes due to resonant charge variations that develop over a comparable time span. ¿ American Geophysical Union 1991