Regularity of Cauchy horizons in S2 x S1 Gowdy spacetimes

We study general S2 x S1 Gowdy models with a regular past Cauchy horizon and prove that a second (future) Cauchy horizon exists, provided that a particular conserved quantity J is not zero. We derive an explicit expression for the metric form on the future Cauchy horizon in terms of the initial data on the past horizon and conclude the universal relation ApAf = (8πJ)2 where Ap and Af are the areas of past and future Cauchy horizons respectively.