  The tadpole cancellation in the unoriented Liouville theory is discussed.
Using two different methods -- the free field method and the boundary-crosscap
state method, we derive one-loop divergences. Both methods require two
D1-branes with the symplectic gauge group to cancel the orientifold tadpole
divergence. However, the finite part left is different in each method and this
difference is studied. We also discuss the validity of the free field method
and the possible applications of our result.
