Innermost stable circular orbit of charged particles in Reissner-Nordström, 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-Nordstr\"om 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 $|qQ|$ in the case of attractive Coulomb interaction $qQl0$. 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-Nordstr\"om and Kerr-Newman spacetime for $qQ\ensuremath{\ge}1$. 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 $qQ\ensuremath{\ge}1+{Q}^{2}$ without dilaton-coupling and $qQ\ensuremath{\ge}1+3/2{\text{ }Q}^{2}$ with dilaton coupling $\ensuremath{\alpha}=1$.