Innermost stable circular orbit of charged particles in Reissner-Nordstrom, Kerr-Newman, and Kerr-Sen spacetimes

In this article we study the innermost stable circular orbit (ISCO) of electrically charged particles in the electrically charged Reissner-Nordstrom spacetime, the Kerr-Newman spacetime and the Kerr-Sen spacetime. We find that the radius of the ISCO increases with an increasing particle-black hole charge product vertical bar qQ vertical bar in the case of attractive Coulomb interaction qQ < 0. For repulsive Coulomb interaction, the ISCO radius first decreases to a minimum and then increases again, until it diverges as the charge product approaches one. If the charge Q of the black hole is very small, the minimum of the ISCO radius lies at qQ = 0. Repulsive and attractive Coulomb interactions will always increase the ISCO radius in this limit. Stable bound orbits of charged particles cease to exist in the Reissner-Nordstrom and Kerr-Newman spacetime for qQ >= 1. In the Kerr-Sen spacetime the limiting case depends on the charge of the black hole and if dilaton coupling is applied to the test particle. We find qQ >= 1 + Q(2) without dilaton-coupling and qQ >= 1 + Q(2) with dilaton coupling alpha = 1.