“…The covariance structure (7) has the so-called product form. Besides the model with the simplest product covariance structure-the Brownian motion with a time-dependent drift [4], [16]-we can find the product covariance structure also in other design problems, see, for instance, [3], [5], [10]. In particular, H a r m a n andŠ t u l a j t e r [3], [5] analysed the autonomous nonstationary Ornstein-Uhlenbeck process discussed in Introduction, which in its original form coincides with the SDE (1) with a constant volatility σ(t) ≡ σ, and assumed the corresponding linear regression model to have a more general response function of the form E[X t ] = (a 1 + b 1 e −κt )θ 1 + (a 2 + b 2 e −κt )θ 2 with a = (a 1 , a 2 ) ′ and b = (b 1 , b 2 ) ′ linearly independent.…”