Progenitor, Precursor and Evolution of the Dusty Remnant of the Stellar
  Merger M31-LRN-2015
  M31-2015-LRN is a likely stellar merger discovered in the Andromeda Galaxy in
2015. We present new optical to mid-infrared photometry and optical
spectroscopy for this event. Archival data shows that the source started to
brighten $\sim$2 years before the nova event. During this precursor phase, the
source brightened by $\sim$3 mag. The lightcurve at 6 and 1.5 months before the
main outburst may show periodicity, with periods of 16$\pm$0.3 and 28.1$\pm$1.4
days respectively. This complex emission may be explained by runaway mass loss
from the system after the binary undergoes Roche-lobe overflow, leading the
system to coalesce in tens of orbital periods. While the progenitor spectral
energy distribution shows no evidence of pre-existing warm dust in system, the
remnant forms an optically thick dust shell at $\sim$4 months after the
outburst peak. The optical depth of the shell increases dramatically after 1.5
years, suggesting the existence of shocks that enhance the dust formation
process. We propose that the merger remnant is likely an inflated giant
obscured by a cooling shell of gas with mass $\sim0.2$ M$_{\odot}$ ejected at
the onset of the common envelope phase.
