  We study analytically the disruptive effect of instantaneous gas removal from
a cluster containing O stars.
  We setup an iterative calculation based on the stellar velocity distribution
function to compute the fraction of stars that remain bound once the cluster
has ejected the gas and is out of equilibrium. We show that the stellar bound
fraction is a function of the initial cluster distribution function as well as
the star formation efficiency, $\epsilon$, taken constant throughout the
cluster.
  The case of the Plummer sphere is dealt with in greater details. We find for
this case that up to ~ 50% of the stars may remain bound when $\epsilon$
assumes values < 1/2, contrary to expectations derived from the virial theorem.
The fraction of bound stars is expressed algebraically for polytropic
distribution functions.
