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Nova Sco 2008 (=V1309 Sco) is an example of a V838 Mon type eruption ratherthan a typical classical nova. This enigmatic object was recently shown to haveresulted from the merger of two stars in a contact binary. It is the firststellar merger that was identified to be undergoing a common envelopetransient. To understand the properties of its binary progenitor, thepre-outburst light curves were analyzed by using the W-D method. Thephotometric solution of the 2002 light curve shows that it is a deep contactbinary (f = 89.5(+-40.5)%) with a mass ratio of 0.094. The asymmetry of thelight curve is explained by the presence of a dark spot on the more massivecomponent. The extremely high fill-out factor suggests that the merging of thecontact binary is driven by dynamical mass loss from the outer Lagrange point.However, the analysis of the 2004 light curve indicates that no solutions wereobtained even at an extremely low mass ratio of q = 0.03. This suggests thatthe common convective envelope of the binary system disappeared and thesecondary component spiraled into the envelope of the primary in 2004. Finally,the ejection of the envelope of the primary produced the outburst.

A low-mass-ratio and deep contact binary as the progenitor of the merger V1309 Sco