In the present study, we obtained two primary minimum times and one
secondary minimum time. Minimum times that have been obtained ever since the system was found
to be an eclipsing binary are given in Table 1.
O-C (I) deviations are plotted in Fig. 2.
O-C deviations show a sine-like scattering around the linear fit. This sine-like curve, although it has not shown a complete cycle yet, seems to be nearing its one cycle completion.
Such a variation could be accounted for in three different ways:
1. Mass transfer. Components of HW Vir are sdB and low mass main sequence stars. It is known that in such a system, where mass accretion occurs from a low mass star to a high mass one, the orbital period increases. Nevertheless, it is also known from the light curve analysis that in the HW Vir system, the low mass star has not filled its Roche lobe yet (see Sect. 4). For this reason it is not reasonable to account for the O-C deviations in terms of mass transfer.
2. Magnetic braking. It was stated by Patterson (1984) that no matter how small the rate of mass-loss is, magnetic braking would eventually cause a decrease in period. However, in the HW Vir system, since the low mass star is very much smaller than its own Roche lobe it becomes unrealistic to expect that the low mass star will lose mass through stellar wind. Besides, the variation of the orbital period of HW Vir shows an uniform decreasing pattern which would have shown increasing and decreasing
portions otherwise. Therefore, neither magnetic braking is the likely mechanism that causes O-C deviations in the system.
3. A third body. Sinusoidal variations in O-C may be attributable either to an orbital motion around a third body or to apsidal motion. But the observational facts such as circular orbit inferred from the radial velocities (Hilditch et al. 1996) are the strongest evidence that would compel one to rule out the apsidal motion as a possible mechanism for period change. What remains behind is the possibility of orbital motion around a third body.
The additional time delay of any observed eclipse due to orbiting around a third-body can be represented by,
c = 2.590 1010 km d-1 is the velocity of light. In this case
the resulting eclipse ephemeris is given by, where t0 is the starting epoch, E is the integer eclipse cycle number. We have
Irwin's (1952, 1959)
definition for the zero of the light-time effect.
The linear least squares solution
was applied in order to obtain the best fit and the standard deviations of the
parameters calculated. The results are given in Table 2.
The final parameters, given in Table 2, were used to obtain the calculated light-time values and the computed light-times were plotted in Fig. 3 along with the observed values.
Using the asini value given in Table 2, the mass function has been computed to be . Assuming a combined mass of for the eclipsing pair the mass of the third body can be evaluated from the mass function depending on the inclination of the third-body orbit with respect to the plane of the sky. The mass of the third component was found to be for inclination 90, and for the inclination 60. For the inclination above 17 the mass computed by us would lead to a sub-stellar mass. Therefore, such a low mass of tertiary component would make it too faint for direct detection. If we assume the inclination is about 17 the mass of the third body would be . The visual magnitude of such a body would be for the distance of 141 pc.
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