The new discovered short-period close binary star , XY LMi , was monitored photometrically since 2006 .
It is shown that the light curves are typical EW-type and show complete eclipses with an eclipse duration of about 80 minutes .
By analyzing the complete B , V , R , and I light curves with the 2003 version of the W-D code , photometric solutions were determined .
It is discovered that XY LMi is a high fill-out , extreme mass ratio overcontact binary system with a mass ratio of q = 0.148 and a fill-out factor of f = 74.1 \% , suggesting that it is on the late evolutionary stage of late-type tidal-locked binary stars .
As observed in other overcontact binary stars , evidence for the presence of two dark spots on both components are given .
Based on our 19 epoches of eclipse times , it is found that the orbital period of the overcontact binary is decreasing continuously at a rate of dP / dt = -1.67 \times { 10 ^ { -7 } } days/year , which may be caused by the mass transfer from the primary to the secondary or/and angular momentum loss via magnetic stellar wind .
The decrease of the orbital period may result in the increase of the fill-out , and finally , it will evolve into a single rapid-rotation star when the fluid surface reaching the outer critical Roche Lobe .