Photoelectric observations in B and V bands of DD Mon were made
on 7 nights in January and February 1996, with the 1.0-m telescope
at Yannan Observatory, Academia Sinica. An EMI 6256B photomultiplier
was used in the observations. The B and V filters approximate
Johnson's standard UBV system. An integration time of 20 seconds
was chosen for both stars and sky. HD 292321 and HD 48867 were chosen
as comparison star and check star, the same as used by
Yamasaki et al. (1990). Nightly extinction coefficients were
determined from observations of the comparison star. The magnitude
differences between the check star and comparison star were constant
within probable errors of 
0.012 mag (V) and
0.013 mag (B) during
the observations. A total of 403 points were obtained in yellow and
357 points in blue. Differential magnitudes V and B, in the sense of
the variable minus the comparison, are presented in Table 1 for V
band and Table 2 for B band.
From these observations, three times of primary minima were
derived by using the quadratic fitting method, and are listed in
Table 3 (click here). Yamasaki et al. (1990) did not publish their times of
light minima. They used Wachmann's light elements to compute the
phases of their observations and applied the correction of
-0.108 P in their calculations. In this study, we have calculated
a new ephemeris of DD Mon by combining our three times of
light minima with the four times of light minima determined from
yamasaki et al.'s (1990) observations, as
| JD.Hel. | Errors(days) | E |  | Observers |
| 2446411.3042 | 0.0012 | -6494.5 | +0.0007 | Yamasaki et al. |
| 2446420.1085 | 0.0009 | -6479 | +0.0008 | Yamasaki et al. |
| 2446443.1111 | 0.0009 | -6438.5 | -0.0013 | Yamasaki et al. |
| 2446443.9643 | 0.0019 | -6437 | -0.0001 | Yamasaki et al. |
| 2450100.2908 | 0.0004 | 0 | +0.0017 | present |
| 2450124.1447 | 0.0004 | 42 | -0.0011 | present |
| 2450128.1213 | 0.0004 | 49 | -0.0006 | present |
The values listed in Table 3 (click here), were calculated with the new
ephemeris. The observed times of light minima are too few to
study any period change of the system. More observations for DD Mon
will be necessary to determine the period behaviour of this system.
The light variations of DD Mon relative to HD 292321 are shown
in Fig. 1 as open circles. To compare with the 1985/1986
observations published by Yamasaki et al. (1990), we also
show their observations in Fig. 1 as filled circles. It is
very clear that the light curves of DD Mon have changed in the
time interval between 1985/1986 and 1996. The maximum brightness
of our light curves was fainter by 0.03 mag (B) and 0.08 mag (V)
and the minimum brightness was also fainter by 0.08 mag (B) and
0.12 mag (V) as compared with the 1985/1986 observations. The
photometric asymmetries (O'Connell effect) on Yamasaki et al.'s
(1990) light curves are not seen on our light curves.