Context : M31N 2015-01a ( or M31LRN 2015 ) is a red nova that erupted in January 2015 – the first event of this kind observed in M31 since 1988 .
Very few similar events have been confirmed as of 2015 .
Most of them are considered to be products of stellar mergers .

Aims : Results of an extensive optical monitoring of the transient in the period January-March 2015 are presented .

Methods : Eight optical telescopes were used for imaging .
Spectra were obtained on BTA , GTC and the Rozhen 2m telescope .

Results : We present a highly accurate 70 d lightcurve and astrometry with a 0.05″uncertainty .
The color indices reached a minimum 2-3 d before peak brightness and rapidly increased afterwards .
The spectral type changed from F5I to F0I in 6 d before the maximum and then to K3I in the next 30 d. The luminosity of the transient was estimated to 8.7 ^ { +3.3 } _ { -2.2 } \times 10 ^ { 5 } L _ { \odot } during the optical maximum .

Conclusions : Both the photometric and the spectroscopic results confirm that the object is a red nova , similar to V838 Monocerotis .