  We present a method for determining the free energy of coexisting states from
irreversible work measurements. Our approach is based on a fluctuation relation
that is valid for dissipative transformations in partially equilibrated
systems. To illustrate the validity and usefulness of the approach, we use
optical tweezers to determine the free energy branches of the native and
unfolded states of a two-state molecule as a function of the pulling control
parameter. We determine, within 0.6 kT accuracy, the transition point where the
free energies of the native and the unfolded states are equal.
