The Evolution of Luminous Red Nova AT 2017jfs in NGC 4470
  We present the results of our photometric and spectroscopic follow-up of the
intermediate-luminosity optical transient AT 2017jfs. At peak, the object
reaches an absolute magnitude of Mg=-15.46+-0.15 mag and a bolometric
luminosity of 5.5x10^41 erg/s. Its light curve has the double-peak shape
typical of Luminous Red Novae (LRNe), with a narrow first peak bright in the
blue bands, while the second peak is longer lasting and more luminous in the
red and near-infrared (NIR) bands. During the first peak, the spectrum shows a
blue continuum with narrow emission lines of H and Fe II. During the second
peak, the spectrum becomes cooler, resembling that of a K-type star, and the
emission lines are replaced by a forest of narrow lines in absorption. About 5
months later, while the optical light curves are characterized by a fast linear
decline, the NIR ones show a moderate rebrightening, observed until the
transient disappeared in solar conjunction. At these late epochs, the spectrum
becomes reminiscent of that of M-type stars, with prominent molecular
absorption bands. The late-time properties suggest the formation of some dust
in the expanding common envelope or an IR echo from foreground pre-existing
dust. We propose that the object is a common-envelope transient, possibly the
outcome of a merging event in a massive binary, similar to NGC4490-2011OT1.
