FO Virginis - A pre-contact binary

The star FO Vir, a recently discovered eclipsing binary with a period of 0.776 days, has been observed photometrically and spectroscopically. Light-curve and line-profile synthesis shows that FO Vir is semidetached with a mass ratio of 0.15 + or - 0.02. The A7 V primary component very nearly fills its Roche lobe. Radial velocities from the observed and synthesized profiles have been measured by cross correlation. The theoretical radial-velocity curve fits the observations well, to yield a mass of 1.9 + 0.9 or - 0.6 solar mass for the primary and 0.29 + 0.08 or - 0.06 solar mass for the secondary. The mean density is 0.17 + or - 0.02 g/cu cm for the primary, 0.33 + or - 0.03 g/cu cm for the secondary, with log g about 3.9 and Mv about 1.7 for the primary. This suggests the primary has evolved off the zero-age main sequence, while the secondary has a density that shows that for its mass it is highly evolved. The period and mass ratio are typical of the more extreme A-type W UMa systems, which suggests that FO Vir will evolve into a contact binary of this type. Short-period semidetached binaries may be themore » progenitors of at least some W UMa systems. 46 references.« less