Luminous Red Novae: Stellar Mergers or Giant Eruptions?
  We present extensive datasets for a class of intermediate-luminosity optical
transients known as "luminous red novae" (LRNe). They show double-peaked light
curves, with an initial rapid luminosity rise to a blue peak (at -13 to -15
mag), which is followed by a longer-duration red peak that sometimes is
attenuated, resembling a plateau. The progenitors of three of them
(NGC4490-2011OT1, M101-2015OT1, and SNhunt248), likely relatively massive blue
to yellow stars, were also observed in a pre-eruptive stage when their
luminosity was slowly increasing. Early spectra obtained during the first peak
show a blue continuum with superposed prominent narrow Balmer lines, with P
Cygni profiles. Lines of Fe II are also clearly observed, mostly in emission.
During the second peak, the spectral continuum becomes much redder, Halpha is
barely detected, and a forest of narrow metal lines is observed in absorption.
Very late-time spectra (~6 months after blue peak) show an extremely red
spectral continuum, peaking in the infrared (IR) domain. Halpha is detected in
pure emission at such late phases, along with broad absorption bands due to
molecular overtones (such as TiO, VO). We discuss a few alternative scenarios
for LRNe. Although major instabilities of single massive stars cannot be
definitely ruled out, we favour a common envelope ejection in a close binary
system, with possibly a final coalescence of the two stars. The similarity
between LRNe and the outburst observed a few months before the explosion of the
Type IIn SN 2011ht is also discussed.
