  An expanding shell is accelerated outward by radiation from the remnant star
and slowed by ram pressure and accretion as it plows into the interstellar
medium. We set up the general relativistic equations of motion for such a
shell. In the non-relativistic limit, the reduce to the standard Ostriker-Gunn
equations. Furthermore, it is shown that the motion equation can be integrated
in general, reducing in this way the study of the dynamics of a shell to a set
of four coupled first order differential equations, solvable up to quadrature
