  Stellar mergers are expected to take place in numerous circumstences in the
evolution of stellar systems. In particular, they are considered as a plausible
origin of stellar eruptions of the V838 Mon type. V1309 Sco is the most recent
eruption of this type in our Galaxy. The object was discovered in September
2008. Our aim is to investigate the nature of V1309 Sco. V1309 Sco has been
photometrically observed in course of the OGLE project since August 2001. We
analyse these observations in different ways. In particular, periodogram
analyses were done to investigate the nature of the observed short term
variability of the progenitor. We find out that the progenitor of V1309 Sco was
a contact binary with an orbital period of ~1.4 day. This period was decreasing
with time. Similarly the light curve of the binary was also evolving,
indicating that the system evolved toward its merger. The violent phase of the
merger, marked by the systematic brightenning of the object, started in March
2008, i.e. half a year before the outburst discovery. We also investigate the
observations of V1309 Sco during the outburst and the decline and show that
they can be fully accounted for within the merger hypothesis. For the first
time in the literature we show, from direct observations, that contact binaries
indeed end up by merging into a single object, as it was suggested in numerous
theoretical studies of these systems. Our study also shows that stellar mergers
indeed result in eruptions of the V838 Mon type.
