BSN: Light Curve Modeling and Orbital Evolution of the Total-eclipse Contact Binary EZ Oct

Abstract We present the first detailed multiband ( BVR c I c and TESS) photometric analysis of the short-period binary EZ Oct. This study combines ground-based observations conducted at a Southern Hemisphere observatory in Argentina with data from the TESS mission. Investigating the orbital period variations of EZ Oct reveals a steadily increasing period consistent with a quadratic trend. We present a new ephemeris and estimate the mass transfer rate as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>=</mml:mo> <mml:mn>1.353</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>8</mml:mn> </mml:mrow> </mml:msup> </mml:math> M ⊙ yr −1 , indicating ongoing conservative mass transfer from the less massive to the more massive star. Light curve modeling was performed using the PHOEBE Python code in conjunction with the MCMC approach, and the inclusion of a cold starspot was required to achieve an adequate fit. Absolute parameters were estimated using Gaia DR3 parallax and astrophysical equations. Our analysis shows that EZ Oct is a total-eclipse contact binary with a mass ratio of 1.969, a fillout factor of 0.106, and an inclination of 82 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 13. Based on the stellar masses and temperatures of the components, the target system belongs to the W-subtype of contact binaries. The positions of the component stars were displayed on the mass—luminosity and mass—radius diagrams to illustrate their evolutionary status. Moreover, we investigated the relationship between orbital period and stellar luminosity in contact binary stars using a sample of 461 systems with P &lt; 0.5 day. We highlight the position of EZ Oct in the mass ratio—inclination parameter space, showing that it lies within the densely populated region of contact binaries.