  This paper investigates the recent stellar merger transient M31LRN 2015 in
the Andromeda galaxy. We analyze published optical photometry and spectroscopy
along with a Hubble Space Telescope detection of the color and magnitude of the
pre-outburst source. The transient outburst is consistent with dynamically
driven ejecta at the onset of a common envelope episode, which eventually leads
to the complete merger of a binary system. The light curve appears to contain
two components: first $\sim10^{-2} M_\odot$ of fast ejecta driven by shocks at
the onset of common envelope, and later, $\sim0.3 M_\odot$ of further ejecta as
the secondary becomes more deeply engulfed within the primary. Just prior to
merger, we find that the primary star is a $3-5.5 M_\odot$ sub-giant branch
primary star with radius of $30-40R_\odot$. Its position in the color-magnitude
diagram shows that it is growing in radius, consistent with a picture where it
engulfs its companion. By matching the properties of the primary star to the
transient outburst, we show that the optical transient lasts less than ten
orbits of the original binary, which had a pre-merger period of $\sim 10$ days.
We consider the possible orbital dynamics leading up to the merger, and argue
that if the system merged due to the Darwin tidal instability it implies a
lower mass main sequence companion of $0.1-0.6M_\odot$. This analysis
represents a promising step toward a more detailed understanding of flows in
common envelope episodes through direct observational constraints.
