  Orbifold branes in string theory are investigated, and the general field
equations on the branes are given explicitly for type II and heterotic string.
It is shown that the effective cosmological constant on each of the two branes
can be easily lowered to its current observational value, using large extra
dimensions. This is also true for type I string. The radion stability is
studied by using the Goldberger-Wise mechanism, and shown explicitly that it is
stable. Therefore, brane world of string theory provides a viable and built-in
mechanism for solving the long-standing cosmological constant problem. Applying
the formulas to cosmology, we obtain the generalized Friedmann equations on
each of the two branes.
