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 , H \alpha is barely detected , and a forest of narrow metal lines is observed in absorption .
Very late-time spectra ( \sim 6 months after blue peak ) show an extremely red spectral continuum , peaking in the infrared ( IR ) domain .
H \alpha 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 can not 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 .