Physical Nature and Orbital Behavior of the Eclipsing System UZ Leonis

TL;DR

This study presents detailed photometric and orbital analysis of UZ Leo, revealing it as a high-fill overcontact binary with a third component, period variations, and magnetic activity effects.

Contribution

It provides the first comprehensive analysis combining light curves, radial velocities, and eclipse timing to characterize the physical and orbital properties of UZ Leo, including third body detection.

Findings

UZ Leo is a totally-eclipsing A-subtype overcontact binary with 76% fill-out.

The orbital period shows a long-term increase and a 139-year oscillation.

A third component with a mass of 0.30 M_sun likely causes the light-time effect.

Abstract

New CCD photometric observations of UZ Leo were obtained between February 2012 and April 2013, and on February 2017. Its physical properties were derived from detailed analyses of our light curves and existing radial velocities. The results indicate that this system is a totally-eclipsing A-subtype overcontact binary with both a high fill-out factor of 76 \% and a third light source contributing 12 \% light in the $B$ bandpass, 10 \% in $V$, and 7 \% in $R$. The light residuals between observations and theoretical models are satisfactorily fitted by adopting a magnetic cool spot on the more massive primary star. Including our 12 measurements, a total of 172 eclipse times were used for ephemeris computations. We found that the orbital period of UZ Leo has varied due to a periodic oscillation superposed on an upward parabolic variation. The observed period increase at a rate of $+3.49 \times 10^{-7}$ d yr$^{-1}$ can be plausibly explained by some combination of non-conservative mass transfer from the secondary to the primary component and angular momentum loss due to magnetic braking. The period and semi-amplitude of the oscillation are about 139 yrs and 0.0225 d, respectively, which is interpreted as a light-time effect due to a third component with a mass of $M_3 \sin i_3$ = 0.30 M$_\odot$. Because the third lights of 7$-$12 \% indicate that the circumbinary object is very overluminous for its mass, it would possibly match a white dwarf, rather than an M-type main sequence.