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V Sagittae shows quasi-periodic optical high (soft X-ray off) and low (softX-ray on) states with the total period of ~300 days. A binary model ispresented to explain orbital light curves both for the high and low states aswell as the transition mechanism between them. The binary model consists of awhite dwarf (WD), a disk around the WD, and a lobe-filling main-sequence (MS)companion. In the optical high state, the mass transfer rate to the WD exceedsthe critical rate of ~1 x 10^{-6} Msun/yr, and the WD blows an optically thick,massive wind. Surface layers of the disk are blown in the wind and the disksurface extends to the companion or over. As a result, optical luminosity ofthe disk increases by a magnitude because of its large irradiation effect. Themassive wind completely obscures soft X-rays. This corresponds to the opticalhigh/soft X-ray off state. The transition between optical high and low statesis driven by an attenuation of the mass transfer from the secondary. As themass supply stops, the WD wind weakens and eventually stops. The disk shrinksto a Roche lobe size and the optical magnitude drops. This phase corresponds tothe optical low/soft X-ray on state. This cycle is repeated like a limit cycle.The WD can grow in mass at the critical rate and eventually reach theChandrasekhar mass limit. This process is called ``accretion wind evolution,''which is a key evolutionary process in a recently developed evolutionaryscenario of Type Ia supernovae. This evolutionary process was first confirmedin the LMC supersoft X-ray source RX J0513.9$-$6951. Thus, V Sge is the secondexample of accretion wind evolution.Comment: to appear in ApJ, 33 pages including figure

A Limit Cycle Model For Long‐Term Optical Variations of V Sagittae: The Second Example of Accretion Wind Evolution