  We consider the Dirichlet Laplacian in a straight planar strip perturbed by a
bounded periodic symmetric operator. We prove the classical Bethe-Sommerfeld
conjecture for this operator, namely, that this operator has finitely many gaps
in its spectrum provided a certain special function written as a series
satisfies some lower bound. We show that this is indeed the case if the ratio
of the period and the width of strip is less than a certain explicit number,
which is approximately equal to 0.10121. We also find explicitly the point in
the spectrum, above which there is no internal gaps. We then study the case of
a sufficiently small period and we prove that in such case the considered
operator has no internal gaps in the spectrum. The conditions ensuring the
absence are written as certain explicit inequalities.
