  We consider a Schroediner operator in a periodic system of strip-like domains
coupled by small windows. As the windows close, the domain decouples into an
infinite series of identical domains. The operator similar to the original one
but on one copy of these identical domains has an essential spectrum. We show
that once there is a virtual level at the threshold of this essential spectrum,
the windows turns this virtual level into the spectral bands for the original
operator. We study the structure and the asymptotic behavior of these bands.
