1 Introduction

The eclipsing nature of the UV - bright object $\rm BD - 7$$^\circ$ 3477 was discovered by Menzies & Marang (1986). The star, named as HW Vir in the variable stars catalogue, has been observed spectroscopically by Berger & Fringant (1980) and classified as an sdB star. The first UBVRI  light curves were obtained by Menzies & Marang (1986). They analysed the light curves to obtain the physical and geometric properties of the component stars. Using the velocities of the primary component Menzies & Marang (1986) derived the semi-amplitude of the orbit 87.9 km s^-1, a mass function of $0.0082~M_\odot$ and a projected semi-major axis of 1.4 10⁵ km. They suggested the plausible masses of the components to be $0.25~M_\odot$and $0.12~M_\odot$. Later on, Wood et al. (1993) obtained simultaneous UBVR photometry of the short period eclipsing binary HW Vir. Their analyses indicated that the mass of the primary is about $0.50~M_\odot$, which corresponds to sdB stars. Assuming the secondary component is a main sequence star they derived the mass ratio between 0.30 and 0.48. They have also assumed a temperature of T₂=3700 K for the secondary star because it contributes very little light to the total brightness of the system and it fits for a wide range of the mass ratio. Their analyses showed that the system HW Vir consists of an ordinary sdB star with a mass near $0.50~M_\odot$ and a main sequence star with a mass near $0.15~M_\odot$. They have also proposed that the orbital period of the system will decrease as the system loses its angular momentum. Then, the secondary component will fill its own critical Roche lobe and a mass transfer to the sdB component will begin. Therefore the system is the progenitor of a cataclysmic variable. Later on Kilkenny et al. (1994) discussed the period decrease in the sdB eclipsing binary HW Vir based on the data gathered over a nine-year baseline. They concluded that the most plausible mechanism of this decrement in the orbital period was the angular momentum loss via magnetic braking in a weak stellar wind. Hilditch et al. (1996) searched the reflection effect in the system HW Vir. As it is known that the most conspicuous feature of the light curves of the eclipsing binaries, including a sub-dwarf and a low-mass companion, is the reflection effect which is seen outside the primary eclipse, they obtained the radial velocities of the hot sdB primary by means of an echelle spectrograph. Combining the results yielded by the radial velocity curve with that of the light curve solutions (e.g. Wood et al. 1993) they derived the astrophysical parameters of the components of HW Vir. Because the system HW Vir is a single-lined eclipsing binary they have assumed the mass of hot primary as $0.50~M_\odot$, and obtained a mass of $0.14~M_\odot$ for the low-mass secondary star. However, the temperature of T₂=3700 K derived for this low-mass companion is rather higher than those expected from theoretical models, as acted by Hilditch et al. (1996).

In this study we present the new times of minima and the new UBVR light curves of the exotic binary HW Vir. The main purposes of this paper are to reveal the most plausible mechanism of the orbital period decrease and to obtain geometric and physical parameters of the components which constitute the system.