HW-VIRGINIS - A SHORT-PERIOD ECLIPSING BINARY CONTAINING AN SDB STAR

We have obtained simultaneous UBVR photometry of the binary star HW Vir, a detached system with an orbital period of 2 h 48 min. The light curves show primary and secondary eclipses and a pronounced reflection effect. We analyse the light curves using the Wilson-Devinney code. We deduce that the orbital inclination is 80.6-degrees +/- 0.2-degrees, that the primary star has a temperature in the range 29 000 < T1 < 36 000 K, and that the secondary star has T2 is similar to 3700 K. Using the published amplitude of the radial velocity curve of the primary star, 87.9 +/- 4.8 km s-1, we calculate the possible masses and radii of the two stars. For the primary star we find that 4.9 < log g1 less-than-or-equal-to 5.8. These values of log g1 and T1 are consistent with those of sdB stars. If the primary star has a mass of 0.50 +/- 0.04 M., as is usual for sdB stars, then 0.28 < q < 0.34. If the secondary star lies on the main sequence, then 0.30 < q < 0.48. From the temperature and gravity of the primary star, we find that its distance is in the range 42 < D < 151 pc. The system will begin mass transfer when the orbital period has decreased enough to bring the secondary star into contact with its Roche lobe. For q is similar to 0.3, contact will occur when the orbital period is 109 min, well below the gap in the distribution of orbital periods of cataclysmic variables. If angular momentum is lost by gravitational radiation alone, for q is similar to 0.3, contact will occur in 2 x 10(9) yr.