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7. Conclusions

The study of V 839 Oph shows that it is an A-type W UMa eclipsing system and that the common envelope of the system has a structure with convective "continents'' on a radiative envelope. Examination of our solutions for the period variations and the light curve leads to the following conclusions.

  1. Our solution for the period variation of V 839 Oph indicates that the mass transfer rate from the less massive component to the more massive component is higher than expected for an A-type (Cruddace & Dupree 1984). For this system we find tex2html_wrap_inline2903.
  2. If the sinusoidal tex2html_wrap_inline2191 variations are due to conservative mass transfer in the system, the mass should have flown from the primary to the secondary between 1978-1988, and from the secondary to the primary between 1988-1998. It seems that such a frequent shift (every tex2html_wrap_inline2907 yr) in the direction of mass transfer is not possible dynamically.
  3. One other possible mechanism for the sinusoidal tex2html_wrap_inline2191 variations is a third body in the system. For a hypothetical third body, the period (tex2html_wrap_inline2911) and semi-amplitude (0.0065 d) of the sinusoidal tex2html_wrap_inline2191 variations in Fig. 4 (click here) lead to a small mass function of f(M3)=0.00376 tex2html_wrap_inline2917. We obtain tex2html_wrap_inline2919 with the assumption of circular and co-planar orbits of the third body. The semi-major axis a3 of third body orbit around the center of mass of the triple system is about tex2html_wrap_inline2923. The bolometric absolute magnitude of the third body was found to be tex2html_wrap_inline2925 using the mass-luminosity function; tex2html_wrap_inline2927 for tex2html_wrap_inline2929 (Demircan & Kahraman 1991). The third body, if it exists, should revolve much beyond the outer Lagrangian points of V 839 Oph, if it were to be stable.
  4. Another possible mechanism to explain the sinusoidal period variation of V 839 Oph is a period modulation mechanism described by Applegate (1992) due to magnetically induced deformations in the outer layers of one of the two components. This mechanism can however not be tested at present, since not enough data is available about the level of the minima and maxima of the light curves, to compare with detailed models. Assuming that the primary component is the active component, we estimate, by using Applegate's (1992) formula, that an angular momentum transfer of tex2html_wrap_inline2931 is needed to produce the observed orbital period change in the 19.62 yr cycle that is tex2html_wrap_inline2933. The subsurface mean magnetic field of the primary then should be about 8.21 kG. According to the discussion in photometric solution section, if the presence of magnetic spots could create a long-term light variation which is strongly possible but, unfortunately now, we cannot discuss the possible relation between the long-term light curve variation and their probable connection with the sinusoidal period variation cause of absence enough light curves on published.

    We should admit that in last few years, however the period modulation mechanism seems to be the main cause of such periodicities, the insufficient light curves on literature cannot give permit to reach final conclude in many of W UMa systems, for example, V 839 Oph (the present work), BX And (Demircan et al. 1993), AB And (Demircan et al. 1994b, Kalimeris et al. 1994), DK Cyg (Awadalla 1994) while except well observed systems, for example, SW Lac (Glownia 1986), VW Cep (Karimie 1983) and TZ Boo (Awadalla 1989).

In this paper, we have found that the current period of V 839 Oph increases in time. On top of this, it seems likely that the period also oscillates sinusoidally with a period of tex2html_wrap_inline2937 years. At the moment we cannot give a unique explanation for these oscillations but we have made several suggestions. It is clear that only systematic and continuous photometric and spectroscopic observations help to understand the connection between light curve and the period variation.


We express our thanks to Dr. R.F. Peletier for help with the presentation. We are greatly indebted to the referees, Drs. A. Gimenez and A. Claret for valuable suggestions and helpful comments. Thanks are due to Dr. Z. Müyesseroğlu for helpful running of Wilson-Devinney code. We are grateful to the amateur astronomer, H.E. Kum, for his careful observations and participation to the observation program.

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