UBVR photometry of pre-catacylismic binaries: HW Virginis

Abstract

New UBVR light curves of the eclipsing binary HW Vir have been obtained. All the published times of primary and secondary eclipses have been collected. By adding the new times of the eclipses obtained by us, the general behavior of the O–C changes has been revealed. The updated O–C change seems to show a cyclic character. This behavior may be the result of whether rotation of the apsis connecting the star's centers or revolution of the couple around a third body. The former has been excluded due to the fact that the O–C values of primary and secondary eclipses show similar variation, i.e., not 180° out of phase. Therefore the O–C changes have been analysed under the assumption of the light–time effect. The period of revolution around the third body was found to be 19 years. The time span following the discovery of eclipsing nature of the system covers 69 percent of the period predicted. The B, V and R light curves have been analysed by the WD code and the physical parameters of the components were obtained. The hotter, primary component is an sdB star with a temperature of about 36 000 K, and the cooler one is a late–type main sequence star with a temperature of 3300 K. The physical parameters derived for the secondary component are in accordance with the parameters of the theoretical models. This analysis indicates that the low mass, cooler secondary does not fill its critical Roche lobe. Therefore, mass accretion from the cool main sequence star to the hot sdB primary is not yet expected.