Bivariate survival models with Clayton aging functions

In some recent papers, the authors considered a function B that describes the level curves of an exchangeable bivariate survival function F ¯ . The function B permits the analysis of several “multivariate aging properties” of F ¯ . In this paper, the authors consider survival models characterized by the condition that B is a Clayton copula and analyze a related invariance property. This property concerns the family of level curves of the joint survival function of residual lifetimes, when “ages” are increasing.