  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.
