The causal geodesics in the equatorial plane of a static extremal charged black holes in heterotic string theory are examined with regard to their geodesic stability, and compared with similar geodesics in the non-extremal situation. Extremization of the effective potential for time-like and null circular geodesics implies that in the extremal limit, the radius of ISCO(Inner-most Stable Circular Orbit) $(r_{ ISCO})$, circular photon orbit (CPO) $(r_{ ph})$ and marginally bound circular orbit (MBCO) $(r_{ mb})$ are coincident with the event horizon $(r_{ hor})$ i.e. $r_{ ISCO}=r_{ ph}=r_{ mb}=r_{ hor}=2M$. Since the proper radial distance on a constant time slice both in Schwarzschild and Painlev\'{ e}-Gullstrand coordinates become zero, thus these three orbits indeed coincide with the \emph{ null geodesic generators of the event horizon}. This strange behavior is quite different from the static, spherically symmetric extremal Reissner Nordstr{ \o}m black hole.
Categories: gr-qc hep-th
