Cauchy horizon end points and differentiability

Cauchy horizons are shown to be differentiable at end points where only a single null generator leaves the horizon. A Cauchy horizon fails to have any null generator end points on a given open subset iff it is differentiable on the open subset and also iff the horizon is (at least) of class C-1 on the open subset. Given the null convergence condition, a compact horizon which is of class C-2 almost everywhere has no end points and is (at least) of class C-1 at all points. (C) 1998 American Institute of Physics. [S0022-2488(98)00911- 6].