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We present extensive datasets for a class of intermediate-luminosity opticaltransients known as "luminous red novae" (LRNe). They show double-peaked lightcurves, with an initial rapid luminosity rise to a blue peak (at -13 to -15mag), which is followed by a longer-duration red peak that sometimes isattenuated, resembling a plateau. The progenitors of three of them(NGC4490-2011OT1, M101-2015OT1, and SNhunt248), likely relatively massive blueto yellow stars, were also observed in a pre-eruptive stage when theirluminosity was slowly increasing. Early spectra obtained during the first peakshow a blue continuum with superposed prominent narrow Balmer lines, with PCygni profiles. Lines of Fe II are also clearly observed, mostly in emission.During the second peak, the spectral continuum becomes much redder, Halpha isbarely detected, and a forest of narrow metal lines is observed in absorption.Very late-time spectra (~6 months after blue peak) show an extremely redspectral continuum, peaking in the infrared (IR) domain. Halpha is detected inpure emission at such late phases, along with broad absorption bands due tomolecular overtones (such as TiO, VO). We discuss a few alternative scenariosfor LRNe. Although major instabilities of single massive stars cannot bedefinitely ruled out, we favour a common envelope ejection in a close binarysystem, with possibly a final coalescence of the two stars. The similaritybetween LRNe and the outburst observed a few months before the explosion of theType IIn SN 2011ht is also discussed.

Luminous Red Novae: Stellar Mergers or Giant Eruptions?