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:
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 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 are:
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.