

4 Relativistic Binaries
Relativistic binaries are binary systems with at least one
degenerate or collapsed object and an orbital period such that they
will be brought into contact within a Hubble time. (Note that this
definition also includes binaries which are already in contact.)
Outside of dense stellar clusters, most relativistic binary systems
arise from primordial binary systems whose evolution drives them to
tight, ultracompact orbits. The dynamical processes in globular
clusters can drive wide binary systems toward short orbital periods
and can also insert degenerate or collapsed stars into relativistic
orbits with other stars. Before addressing specific evolutionary
scenarios, we will present the generic features of binary evolution
that lead to the formation of relativistic binaries.

