  In this paper, we show the additive separation of the Hamilton-Jacobi
equation, present the 4-velocity of the test particles, and attempt to find the
equatorial circular photon orbit (ECPO) in the Kerr-Sen-Taub-NUT (KSTN)
solution of the low energy limit of heterotic string theory. In the process of
trying to find the ECPO, we instead encounter a ring singularity that could be
outside the horizon of the KSTN black hole.
