Our analysis, with W-D method, of the corrected yellow light curve of AU Mon,
obtained by Lorenzi (L80) yielded the most probable value of the photometric
mass ratio, q=0.1985. This value is found, within observational errors, to be
equal to the spectroscopic mass ratio derived by Popper (1989) from H lines.
Hence we conclude that the presently derived absolute elements are most
reliable. A comparison of the positions of the primary and secondary
components on the Log 
, versus 
, 
and 
relations of the
normal main sequence stars (Andersen 1991) indicate that while the primary
component is having normal luminosity, bigger size and lower temperature, the
secondary component is having higher luminosity, bigger size and normal
temperature, in comparison to stars of similar mass. The normal HR
diagram (Andersen 1991) shows that while the primary component is near but
above the main sequence (brighter by 
), the secondary is far above it
and is overluminous by about 
. In this respect, the secondary component
shares the common property of overluminosity of the secondaries of the semi
detached systems (Sarma et al. 1996). From the properties of the components
of AU Mon, we conclude that it is a typical Algol type binary except that
the eclipse light variations are superposed by intrinsic brightness variations.
We suggest that a detailed study of the nature of the intrinsic variation in
AU Mon is very important. According to Peters (1994) this variation is caused
by the periodic changes in the rate of mass transfer from the secondary due
to pulsations about its Roche surface which, in turn, would cause changes
in the temperature of the mass accreting region around the primary.
Alternately, we suggest that this variation may be due to the precession of
the gaseous disc around the primary caused by the gravitational perturbations
of the secondary component.
Another point to notice is that while the spectral type of the secondary
component as given by Dr. Morgan (in Sahade & Cesco 1945) is F0, the present
study suggests it to be a F9-G0 star. Lorenzi (1983) suggests that early G is
appropriate from photometry. This large discrepancy of about one spectral
class may be attributed partly to the fact that we have observations in one
pass band only. More importantly, corrections for the long term variations were
not applied to the observations during the eclipse phases (L80) when a part of
the variable source is covered causing an error in the depth of the eclipse.
It is obvious that extensive photoelectric observations in as many pass bands
as possible and high dispersion spectroscopic studies are needed for
understanding the long term variations in AU Mon.
Acknowledgements
We are grateful to Professeur K.D. Abhyankar for helpful suggestions. We also thank the referee, Professeur D.M. Popper, for many useful comments. MBKS wishes to thank the Council of Scientific and Industrial Research (CSIR), New Delhi, for financial assistance under the Emeritus Scientists Scheme.