  Recently, by explicit calculations at orders $\alpha',\alpha'^2,\alpha'^3$,
it has been observed that the effective action of string theory at the critical
dimension is independent of the background for the closed spacetime manifolds.
In this paper we speculate that for the open spacetime manifolds, the effective
action is even independent of the character of the boundary, \ie the boundary
couplings for timelike and spacelike boundaries are the same. We support this
proposal by calculating the boundary couplings in the bosonic string theory at
order $\alpha'$ for the spacelike boundary and show that they are the same as
the couplings for the timelike boundary that have been recently found.
