 Kerr ISCO spin limits

For equatorial test-particle circular orbits in the extremal-spin limit, the prograde Kerr black hole innermost stable circular orbit has Boyer-Lindquist radius tending to $GM/c^2$ and specific energy tending to $1/\sqrt{3}$. The retrograde limit has radius $9GM/c^2$ and specific energy $5/(3\sqrt{3})$. The prograde radius approaching the horizon coordinate does not make the massive limiting orbit identical to the null horizon generator.^[ref]