  One of the problems of periodic FPU-chains with alternating masses is whether
significant interactions exist between the so-called (high frequency) optical
and (low frequency) acoustical groups. We show that for $\alpha$-chains with
$4n$ and $8n$ particles ($n= 1, 2, \ldots$) we have significant interactions
caused by external forcing. For $\beta$-chains with $4$ and $8$ particles the
interactions are characterised by parametric forcing, the interactions are
negligible.
