  An effective action is obtained for the area and mass aspect of a thin shell
of radiating self-gravitating matter. On following a mini-superspace approach,
the geometry of the embedding space-time is not dynamical but fixed to be
either Minkowski or Schwarzschild inside the shell and Vaidya in the external
space filled with radiation. The Euler-Lagrange equations of motion are
discussed and shown to entail the expected invariance of the effective
Lagrangian under time-reparametrization. They are equivalent to the usual
junction equations and suggest a macroscopic quasi-static thermodynamic
description.
