Using the light curve of variation of the unknown source in the system as given
by Lorenzi (1982, Table 1 (click here) and Fig. 1 (click here)) we corrected the 183 weighted normals of
Lorenzi (L80; Table 1 (click here)) for the intrinsic brightness variations and analysed them
for elements using Wilson-Devinney (W-D) (1971) method. The number of points in
each of the 183 normals was taken as its weight. For initiating the W-D method,
we used the elements derived by Giuricin et al. (1982) as preliminary elements.
Since the semi detached nature of this system was confirmed by
Giuricin et al.
(1982) and Popper (1989), we used code-5 of the W-D method meant for such
systems and a circular orbit (e=0) was assumed for the analysis. Since the
spectral type of the primary component was reported to be B5
(Sahade &
Cesco 1945; Giuricin et al. 1982; Popper 1989) we assumed a temperature of
K (Allen 1976; Popper 1980;
Schmidt-Kaler 1982) for this component.
As regards the other important parameter, 
, the mass ratio,
Popper (1989) reported two values for 
: one of 
obtained from
He and Mg II lines and the other of 
obtained from H lines.
However, he obtained a unique value of 
for 
from the D lines.
Hence the value of q is ambiguous: it can be either 0.190 (H lines) or 0.213
(He and Mg II lines). In order to find the value of q that gives minimum
from the light curve analysis, we analysed the data using a range of
q values viz.: 0.190, 0.195, 0.1975, 0.20, 0.21 and 0.22, and along with
treated it as a fixed parameter. In addition, the limb darkening
coefficients 
and 
, the albedos, 
and 
; the
gravity darkening coefficients, 
and 
of the primary and
secondary components were also treated as fixed parameters. The following
parameters were treated as adjustable: i, the inclination of the orbit; 
,
the surface potential of the hotter component; 
, the relative monochromatic
luminosity of the hotter component; 
, the mean effective temperature of the
cooler component and l3, the third light. With these fixed and adjustable
parameters, a number of runs of the program (code-5) were made till the sum of
the residuals showed a minimum and the corrections to the parameters
became smaller than their probable errors. The values of obtained
from the analysis for different q values are given in Table 1 (click here).
A free hand drawn curve through a plot of q versus
(Fig. 1 (click here))
showed a
minimum at q=0.1985. Hence we obtained another solution, as before,
with q=0.1985 and
as fixed parameters. In this solution, we treated 
, 
and 
also as adjustable parameters. The results are given in Table 2 (click here). One can notice
that l3 is absent in the solution.
| Parameter | Values | |
 | | |
 |  | |
| *q | 0.1985 | |
 |  | |
| pole |  | |
| point |  | |
 | side |  |
| back | | |
| pole |  | |
| point |  | |
 | side |  |
| back |  | |
 |  | |
 | 0.3410 | |
| l3 |  | |
|
|  | |
|
|  | |
 | 1.0 | |
|
|  | |
 | 0.25 | |
 | 0.08 | |
 | 1.0 | |
 | 1.0 | |
| * Fixed parameters. | ||
and q=0.1985 as fixed parameters The theoretical light curve obtained from the parameters given in Table 2 (click here) is shown as solid line in Fig. 2 (click here).
Figure 2: AU Mon: Light curve in yellow.The solid line is the theoretical
curve obtained from the parameters given in Table 2 (click here). The filled circles represent
the 183 corrected normal points used in the present analysis. The 27 normal points
of Lorenzi (1982) are shown as squares
Here the filled circles represent the corrected 183 normal points. The 27 normal points of Lorenzi (1982, Table 3 (click here)) are shown as squares. Keeping in view the sparse observational coverage at some phases of the light curve, we conclude that the fit of the theoretical curve to the observations is quite satisfactory except in the phase range of 0.88 to 0.96 and 0.04 to 0.12. We attribute this misfit to the presence of gases or gas streams in the system that are favourably situated to absorb some of the light during these phases. The presence of gases or gas streams in AU Mon was evidenced by the spectroscopic studies of Sahade and Cesco (1945), Sahade & Ferrer (1982), Popper (1962), Peters & Polidan (1984) and Peters (1994). Similar misfits and distortions of light curves due to the presence of gases in the semi detached systems of TT Hya (Vivekananda Rao & Sarma 1994), HU Tau (Parthasarathy et al. 1995), R CMa (Sarma et al. 1996), EU Hya (Vivekananda Rao et al. 1996) and RY Gem (Sarma & Vivekananda Rao 1997) were already reported.
| Parameter | Hot Component | Cool Component |
 |  | |
 |  |  |
 |  |  |
 |  |  |
 |  |  |
 |  |  |
 |  |  |
and 
and other parameters from Table 2 (click here)