On analyticity of static vacuum metrics at non-degenerate horizons
  We show that static metrics solving vacuum Einstein equations (possibly with a cosmological constant) are one-sided analytic at non-degenerate Killing horizons. We further prove analyticity in a two-sided neighborhood of "bifurcate horizons".   It is a pleasure to dedicate this work to Prof. Staruszkiewicz, on the occasion of his 65th birthday. 
Piotr T. Chrusciel