  We consider the non-trivial boundary conformal field theory with exactly
marginal boundary deformation. In recent years this deformation has been
studied in the context of rolling tachyons and S-branes in string theory. Here
we study the problem directly from an open string point of view, at one loop.
We formulate the theory of the Z_2 reflection orbifold. To do so, we extend
fermionization techniques originally introduced by Polchinski and Thorlacius.
We also explain how to perform the open string computations at arbitrary
(rational) radius, by consistently constructing the corresponding shift
orbifold, and show in what sense these are related to known boundary states. In
a companion paper, we use these results in a cosmological context involving
decaying branes.
