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New CCD photometric observations of UZ Leo were obtained between February2012 and April 2013, and on February 2017. Its physical properties were derivedfrom detailed analyses of our light curves and existing radial velocities. Theresults indicate that this system is a totally-eclipsing A-subtype overcontactbinary with both a high fill-out factor of 76 \% and a third light sourcecontributing 12 \% light in the $B$ bandpass, 10 \% in $V$, and 7 \% in $R$.The light residuals between observations and theoretical models aresatisfactorily fitted by adopting a magnetic cool spot on the more massiveprimary star. Including our 12 measurements, a total of 172 eclipse times wereused for ephemeris computations. We found that the orbital period of UZ Leo hasvaried due to a periodic oscillation superposed on an upward parabolicvariation. The observed period increase at a rate of $+3.49 \times 10^{-7}$ dyr$^{-1}$ can be plausibly explained by some combination of non-conservativemass transfer from the secondary to the primary component and angular momentumloss due to magnetic braking. The period and semi-amplitude of the oscillationare about 139 yrs and 0.0225 d, respectively, which is interpreted as alight-time effect due to a third component with a mass of $M_3 \sin i_3$ = 0.30M$_\odot$. Because the third lights of 7$-$12 \% indicate that the circumbinaryobject is very overluminous for its mass, it would possibly match a whitedwarf, rather than an M-type main sequence.

Physical Nature and Orbital Behavior of the Eclipsing System UZ Leonis