  Binary stars commonly pass through phases of direct interaction which result
in the rapid loss of mass, energy, and angular momentum. Though crucial to
understanding the fates of these systems, including their potential as
gravitational wave sources, this short-lived phase is poorly understood and has
thus far been unambiguously observed in only a single event, V1309 Sco. Here we
show that the complex and previously-unexplained photometric behavior of V1309
Sco prior to its main outburst results naturally from the runaway loss of mass
and angular momentum from the outer Lagrange point, which lasts for thousands
of orbits prior to the final dynamical coalescence, much longer than predicted
by contemporary models. This process enshrouds the binary in a "death spiral"
outflow, which affects the amplitude and phase modulation of its light curve,
and contributes to driving the system together. The total amount of mass lost
during this gradual phase ($\sim 0.05 M_\odot$) rivals the mass lost during the
subsequent dynamical interaction phase, which has been the main focus of
"common envelope" modeling so far. Analogous features in related transients
suggest that this behavior is ubiquitous.

Solar and Stellar Astrophysics