Given ail order 1 autoregressive process with values in a separable Hilbert
space we show that its empirical covariance operator say C-n, is the empir
ical mean of an autoregressive process which takes its values in the space
of Hilbert-Schmidt operators with a noise which is an Hilbertian martingale
difference. This allows to obtain precise results concerning the asymptoti
c behaviour of C-n and its eigenvalues. (C) 1999 Academie des Sciences/Edit
ions scientifiques et medicales Elsevier SAS.