The first pre-supersoft X-ray binary

We report the discovery of an extremely close white dwarf plus F dwarf main-sequence star in a 12 h binary identified by combining data from the Radial Velocity Experiment survey and the Galaxy Evolution Explorer survey. A combination of spectral energy distribution fitting and optical and Hubble Space Telescope ultraviolet spectroscopy allowed us to place fairly precise constraints on the physical parameters of the binary. The system, TYC 6760-497-1, consists of a hot T-eff similar to 20 000 K, M-WD similar to 0.6 M-circle dot white dwarf and an F8 star (M-MS similar to 1.23 M-circle dot, R-MS similar to 1.3 R-circle dot) seen at a low inclination (i similar to 37 degrees). The system is likely the descendant of a binary that contained the F star and an similar to 2 M-circle dot A-type star that filled its Roche lobe on the thermally pulsating asymptotic giant branch, initiating a common envelope phase. The F star is extremely close to Roche lobe filling and there is likely to be a short phase of thermal time-scale mass transfer on to the white dwarf during which stable hydrogen burning occurs. During this phase, it will grow in mass by up to 20 per cent, until the mass ratio reaches close to unity, at which point it will appear as a standard cataclysmic variable star. Therefore, TYC 6760-497-1 is the first known progenitor of a supersoft source system, but will not undergo a Type Ia supernova explosion. Once an accurate distance to the system is determined by Gaia, we will be able to place very tight constraints on the stellar and binary parameters.FONDECYT [3140585, 3130559, 1120338, 1141269, 1130857]; Proyecto FONDECYT de Iniciacion [11140572]; European Research Council under the European Union's Seventh Framework Programme (FP)/ERC [320964]; Millennium Science Initiative, Chilean ministry of Economy [Nucleus RC130007]; Postdoctoral Science Foundation of China [2013M530470, 2014T70010]; Research Fund for International Young Scientists by the National Natural Science Foundation of China [11350110496]; CONICYT-PCHA/Doctorado Nacional; 'Millenium Institute of Astrophysics (MAS)' of the Millennium Science Initiative, Chilean Ministry of Economy [IC120009]; Ministry for the Economy, Development, and Tourism's Programa Iniciativa Cientifica Milenio [IC 120009]; BASAL CATA [PFB-06]; NASA [NAS 5-26555]; Spanish MICINN [AyA2008-02156]; [13704]SCI(E)ARTICLEsteven.parsons@uv.cl21754-176345