  We present the results of the study of the red nova PSN J14021678+5426205
based on the observations carried out with the Russian 6-m telescope (BTA)
along with other telescopes of SAO RAS and SAI MSU. To investigate the nova
progenitor, we used the data from the Digital Sky Survey and amateur photos
available on the internet. In the period between April 1993 and July 2014, the
brightness of the progenitor gradually increased by 2.2 mag in the V band. At
the peak of the first outburst in mid-November of 2014, the star reached an
absolute visual magnitude of -12.75 mag but was discovered later, in February
2015, in a repeated outburst at the absolute magnitude of -11.65 mag. The
amplitude of the outburst was minimum among the red novae, only 5.6 mag in the
V band. The H alpha emission line and the continuum of a cool supergiant with a
gradually decreasing surface temperature were observed in the spectra. Such
process is typical for red novae, although the object under study showed
extreme parameters: maximum luminosity, maximum outburst duration, minimum
outburst amplitude, unusual shape of the light curve. This event is interpreted
as a massive OB star system components merging accompanied by the formation of
a common envelope and then the expansion of this envelope with minimal energy
losses.
