  We consider time-dependent orbifolds in String Theory and we show that
divergences are not associated with a gravitational backreaction since they
appear in the open string sector too. They are related to the non existence of
the underlying effective field theory as in several cases fourth and higher
order contact terms do not exist. Since contact terms may arise from the
exchange of string massive states, we investigate and show that some three
points amplitudes with one massive state in the open string sector are
divergent on the time-dependent orbifolds. To check that divergences are
associated with the existence of a discrete zero eigenvalue of the Laplacian of
the subspace with vanishing volume, we construct the Generalized Null Boost
Orbifold where this phenomenon can be turned on and off.
