5 Conclusion and discussion

It is known that sdB stars represent part of the final stage of stellar evolution. As is already stated before, HW Vir is the progenitor of cataclysmic binary stars. As soon as the low mass main sequence component fills its Roche lobe the system is expected to become a cataclysmic binary star. HW Vir and the similar eclipsing binaries play a very important role in understanding the latest stages of stellar evolution. In Menzies & Marang (1986) the mass and the temperature of the sdB are $M_1=0.25~M_\odot$ and $26\ 000~$K, respectively. In the same paper, the luminosity of the primary is given as $6.53~L_\odot$. However, Saffer et al. (1994) showed that the mass of a sdB star should be around $0.50~\pm~0.01~M_\odot$. For this reason we adopted the mass of sdB component in HW Vir as $0.50~M_\odot$. Using the mass function of Hilditch et al. (1996) we found the mass of the companion as $0.14~M_\odot$. The values we calculated indicate that the luminosity of the hotter component of HW Vir should be as high as $55.6~L_\odot$. In the diagram developed by Iben & Tutukov (1993) showing the latest stages of low mass stars, the hotter component of HW Vir falls between the evolutionary tracks of stars of $0.38~M_\odot$ and $0.52~M_\odot$. On the other hand temperature, radius and luminosity values of the low-mass component are consistent with the corresponding theoretical values that are calculated on the basis of assumed masses (Dorman et al. 1989), as shown also by Hilditch et al. (1996).

O-C variations of HW Vir obtained from data covering a 19 year period are assumed to be caused by orbital motion about a third body in the system. Based on this assumption, the analysis of the system implies that it is revolving around a third body on an elliptic orbit of the radius of 30.72 10⁶ km with a period of 7040 days. The data that is gathered between the period 1984-1997 indicated that the system has completed 69 per cent of one whole cycle of the orbital motion. Observations of the system in the following years will test the justifiability of the third body hypothesis. The light curve analysis of the system indicates that the low mass component has just filled 80 per cent of its own Roche Lobe.

Acknowledgements

Our sincere thanks go to Dr. C. Ibanoglu; without his unsparing support this work would have not

possibly come to its completion. We also thank Dr. R. Pekünlü who carefully read the manuscript. This study was partly supported by Ege University Science Foundation. This is part of the M.Sc. thesis presented by Ömür Çakirli.