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.