  We propose a new scenario for generating a relic density of non-relativistic
dark matter in the context of heterotic string theory. Contrary to standard
thermal freeze-out scenarios, dark-matter particles are abundantly produced
while still relativistic, and then decouple from the thermal bath due to the
sudden increase of their mass above the universe temperature. This mass
variation is sourced by the condensation of an order-parameter modulus, which
is triggered when the temperature T(t) drops below the supersymmetry breaking
scale M(t), which are both time-dependent. A cosmological attractor mechanism
forces this phase transition to take place, in an explicit class of heterotic
string models with spontaneously broken supersymmetry, and at finite
temperature.
