The process is not stationary for RX(t+τ, t) does not depend only on τ but on t as well. However
the process is cyclostationary with period T0 = 1
2f0 . Note that if X or Y is not of zero mean then
the period of the cyclostationary process is T0 = 1
f0 . The power spectral density of X(t) is
does not depends only on τ .