The concern of this paper is to study a class of nonstationary signals of the form x(t)c(t) where x(t) is a stationary Gaussian stochastic process and c(t) is a deterministic signal. The process x(t) is modeled by an autoregressive (AR) process. The deterministic signal c(t) is a known function of a finite-dimensional unknown vector. Closed-form expressions are derived for the finite-sample Cramér–Rao bound. Algorithms for the maximum likelihood estimation of c(t) and the spectral density of x(t) are developed. The proposed methods are applied to the problem of estimating abrupt change in multiplicative noise.