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4. Conclusion and discussion

Our photometric parameters (Table 5 (click here)) have been combined with Yamasaki et al.'s (1990) spectroscopic elements to obtain absolute values of DD Mon:


eqnarray314

The present solution indicates that DD Mon is a semi-detached system with the secondary component filling its Roche lobe. Yamasaki et al.'s (1990) solution indicated a near-semidetached system with the primary component almost filling its Roche lobe. The photometric parameters are also distinctly different, which may be caused by the following reasons: (1) Yamasaki et al. (1990) and we used different analysis method. They used their own program, while we employed Wilson-Devinney's synthetic code. Generally speaking, the two methods should give the same solutions, but, for binaries with some photometric disturbances, the solution often is not unique; there are two minimal values on the tex2html_wrap_inline984 diagram (Fig. 2), one at q=0.5, the other at q=0.7. As we can see in Fig. 2, Yamasaki et al.'s solution (at q=0.7) appear to be local solutions. (2) The variation of light curves may also contribute to the differences in solutions.

Yamasaki et al.'s (1990) absolute parameters derived from their photometric mass ratio tex2html_wrap_inline992 are:

eqnarray339

These absolute parameters suggest that the components of DD Mon are very undermassive for their radii and luminosities, indicating that the components of DD Mon are giants or subgiants. This is unlikely because the evolution times of components with such small masses are longer than the lifetime of the Galaxy. Yamasaki et al. (1990) proposed that this was caused by mass loss from the system during the course of evolution. Their finding that DD Mon is a detached system is difficult to understand. The absolute parameters determined by using our photometric mass ratio provide a mass for the primary component that is sensibly consistent with the observed spectral type of F5V determined by Yamasaki et al. (1990), and demonstrates that the system is a post-mass-transfer semi-detached binary. The locations of the two components in the mass-radius plane and in the HR diagram are consistent with similar systems reported in Hilditch et al. (1988).

Yamasaki et al. and we used the same comparison star and check star. Our magnitude difference between the check star and the comparison star was equal to that of Yamasaki et al., which indicates that the comparison star was not variable in the period between 1985/1986 and 1996. The variation of maximum light and minimum light may be caused by evolution of the system. The photometric asymmetries (O'Connell effect) on Yamasaki et al.'s (1990) light curves were not seen when we observed. This was seen also in the case of SS Ari (Qingyao Liu et al. 1993), which demonstrated that DD Mon belongs to this kind of binary with variable O'Connell effect. The photometric disturbance are presumably caused by the presence of dark spots variable extent.


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