ISCO and Principal Null Congruences in Extremal Kerr Spacetime
  The effective potential in universal like coordinates$(U, V, \theta, \phi)$, which are smooth across the event horizon is derived and investigated the ISCO(Innermost Stable Circular Orbits) explicitly in these coordinates for extremal Kerr spacetime. Extremization of the effective potential for timelike circular orbit shows that the existence of a stable circular geodesics in the extremal spacetime for direct orbit, precisely {\it on} the event horizon in terms of the radial coordinate which coincides with the \emph{principal null geodesic congruences} of the event horizon. These null geodesic congruences mold themselves to the spacetime curvature in such a way that Weyl conformal tensor and its dual are vanished, that is why they are in fact \emph{doubly degenerate principal null congruences}. 
Partha Pratim Pradhan
gr-qc