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

Separability of the Hamilton-Jacobi equation and equatorial circular photon orbit in Kerr-Sen-Taub-NUT spacetime