Pseudo full likelihood estimation for prospective survival analysis with   a general semiparametric shared frailty model: asymptotic theory
  In this work we present a simple estimation procedure for a general frailty model for analysis of prospective correlated failure times. Earlier work showed this method to perform well in a simulation study. Here we provide rigorous large-sample theory for the proposed estimators of both the regression coefficient vector and the dependence parameter, including consistent variance estimators. 
David M. Zucker, Malka Gorfine, Li Hsu