Convergence rate of linear two-time-scale stochastic approximation

We study the rate of convergence of linear two-time-scale stochastic approximation methods. We consider two-time-scale linear iterations driven by i.i.d. noise, prove some results on their asymptotic covariance and establish asymptotic normality. The well-known result [Polyak, B. T. (1990). Automat. Remote Contr. 51 937.946; Ruppert, D. (1988). Technical Report 781, Cornell Univ. ] on the optimality of Polyak.Ruppert averaging techniques specialized to linear stochastic approximation is established as a consequence of the general results in this paper.