tokens: ratio moving in circular orbits, the filling of the Roche lobe is followed by violent mass transfer by the primary to the secondary and the formation of a common envelope. On the other hand, an equal-mass binary may evolve into a fully contact system of two stars, both of which fill their Roche lobes simultaneously. In the limiting case, violent mass transfer may occur on the hydrodynamic time-scale, with an amount of released energy of the order of the thermal energy of the components or the common envelope: GM 2 / a 510 48 ( M /M ) 2 ( a /R ), where M is the mass of the component or of the common envelope and a is the radius of the common envelope. For 3M common envelope mass and energy from Table 1 , the envelope radius is about 10R . The high energy of the studied explosion is a consequence of compactness and the relatively high mass of the parent system. From this point of view, it is very important to obtain observational evidence for such processes in binary systems and to determine the corresponding physical properties: the released energy, ejected mass and angular momentum. Ultraluminous red novae may be observational manifestations of violent mass transfer in the binary (Blagorodnova etal. 2017 ). We already pointed out that observation of the progenitor of V1309 Sco in the form of a contact binary with a period of 1.7d (Tylenda etal. 2011 ) provides strong evidence for the merger model of LRNe. This conclusion is also corroborated by the approximate occurrence rate of such events one event in 2040yr per galaxy like the Milky Way or M31. To show this, let us use the distribution of binary semimajor axes a (Masevich Tutukov 1981 ). This distribution has the form d N 0.2dlog( a /R ). Systems with 10 a /R 1000 can pass through the common-envelope stage during the second mass exchange stage. According to the initial distribution of binaries over semimajor axes of Masevich Tutukov ( 1981 ), N 0.2dlog( a /R ), the frequency of formation of common envelopes with radii 610R is about 0.04 per year. Systems with larger component separations are less energetic. 5 CONCLUSIONS In this article, we described the discovery of the nova M31LRN and long-term observations of its light curve with the MASTER network of robotic telescopes. It is an important point that the entire observational part of the study was performed on identical telescopes equipped with identical photometers. The resulting light curve agrees fairly well with the independent light curve published by Kurtenkov etal. ( 2015b ). An analysis of the photometric observations and velocity data for the nova M31LRN made using radiative hydrodynamic modelling showed that the rather long observable plateau (50d) could be achieved only with a higher merged stellar mass M 3M and common envelope radius R 10R . The corresponding explosion energy should be lower, 310 50 erg, whereas the total kinetic energy of the ejected envelope is lower by three orders of magnitude. The proposed
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