  V1309 Sco was proposed to be a stellar merger and a common envelope transient
based on the pre-outburst light curve of a contact eclipsing binary with a
rapidly decaying orbital period. Using published data, I show that the period
decay timescale P/Pdot of V1309 Sco decreased from ~1000 to ~170 years in less
than about 6 years, which implies a very high value of second period
derivative. I argue that V1309 Sco experienced an onset of dynamical mass loss
through the outer Lagrange point, which eventually obscured the binary. The
photosphere of the resulting continuous optically-thick outflow expands as the
mass-loss rate increases, explaining the ~200 day rise to optical maximum. The
model yields the mass-loss rate of the binary star as a function of time and
fits the observed light curve remarkably well. It is also possible to
observationally constrain the properties of the surface layers undergoing the
dynamical mass loss. V1309 Sco is thus a prototype of a new class of stellar
transients distinguished by a slow rise to optical maximum that are driven by
dynamical mass loss from a binary. I discuss implications of these findings for
stellar transients and other suggested common envelope events.
