A Photometric Study of the Peculiar Binary Star V Sagittae

High-speed photometric UBV observations of the novalike variable V Sagittae conducted during 1994 and 1995 are reported. The data consist of a total of 152 hr of observations, covering the entire 12.3 hr orbital period. Analysis of the photometry using the new Wilson eclipsing-binary code suggests a model where mass is being transferred from a 0.86 M⊙, evolved, helium-burning star, which fills its Roche lobe, to a 3.48 M⊙ evolved companion. Thirteen new timings of primary eclipse in conjunction with published timings from 1946 to 1996 show that the orbital period is changing at a rate ṗ=—5.1(± 1.0)X10⁻¹⁰. We derive a lower limit to the mass-loss rate from the primary star of Ṁ₁ ~ –4.4(± 0.1)X10⁻⁷ M⊙ yr⁻¹. V Sge appears to be in a short-lived evolutionary state with a lifetime of ~ 2X10⁶ yr.