  In this work we analyze the effects of an external magnetic field on charged
particles on equatorial circular orbits in a Kerr spacetime, both in the black
hole and the naked singularity cases. Understanding these phenomena is of great
importance because equatorial circular orbits are a key ingredient of simple
accretion disc models. We focus on two important magnetic field configurations:
a) a uniform magnetic field aligned with the angular momentum and b) a dipolar
magnetic field. We center our attention on the effect of these external fields
on the marginally bounded and marginally stable equatorial circular orbits
because they give information on observable quantities that could be useful to
determine whether the central object is a black hole or a naked singularity.
Using a perturbative approach we are able to give analytic results and compare
(in the black hole case) with previous results.
