Using our new photometric and spectroscopic observations as well as all available published data , we present a new interpretation of the properties of the peculiar emission-line binary UX Mon .
We conclude that this binary is in a rare phase of fast mass transfer between the binary components prior to the mass ratio reversal .
We firmly establish that the orbital period is secularly decreasing at a rate of \dot { P } = -0.260 seconds per year .
From several lines of reasoning , we show that the mass ratio of the component losing mass to the mass-gaining component q must be larger than 1 and find our most probable value to be q = 1.15 \pm 0.1 .
The BINSYN suite of programs and the steepest descent method were used to perform the final modeling .
We modeled the star as a W Ser star with a thick disk around its primary .
Although the remaining uncertainties in some of the basic physical elements describing the system in our model are not negligible , the model is in fair agreement with available observations .
Only the nature of the light variations outside the primary eclipse remains unexplained .