  The four-form field recently considered by Hawking and Turok couples
naturally to a charged membrane, across which the effective cosmological
constant has a discontinuity. We present instantons for the creation of an open
inflationary universe surrounded by a membrane. They can also be used to
describe the nucleation of a membrane on a pre-existing inflationary
background. This process typically decreases the value of the effective
cosmological constant and may lead to a novel scenario of eternal inflation.
Moreover, by coupling the inflaton field to the membrane, the troublesome
singularities which arise in the Hawking-Turok model can be eliminated.
