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

TL;DR

This study analyzes EZ Oct, a short-period W UMa-type contact binary, using multiband ground-based (BVR_cI_c) and TESS photometry to model its light curve with PHOEBE and to investigate orbital-period variations via O–C analysis. A quadratic O–C trend indicates a secular period increase consistent with conservative mass transfer, yielding a mass-transfer rate of $\dot{M}=1.353\times10^{-8}$ M$_{\odot}$ yr$^{-1}$ and a corresponding period derivative $\dot{P}\approx 1.09\times10^{-3}$ s yr$^{-1}$; the system is characterized as a shallow, high-inclination W-subtype with $q\approx1.97$, $i\approx82.1^{\circ}$, and a fillout factor $f\approx0.106$. The light-curve fit requires a cool spot on the secondary to explain the O'Connell effect, with spot parameters tracked across four TESS sectors, indicating evolving magnetic activity without altering global binary parameters. Absolute stellar parameters are derived from Gaia DR3 parallax, placing the components at $M_1\approx0.45\,M_\odot$ and $M_2\approx0.90\,M_\odot$, among other derived quantities, and revealing EZ Oct's position on $M$–$R$ and $M$–$L$ planes near ZAMS/TAMS tracks. The work updates period–luminosity relations for short-period W UMa binaries and discusses the system's angular momentum and TRO-context implications for its evolutionary trajectory.

Abstract

We present the first detailed multiband (BVR_cI_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 \dot{M}=1.353*10^{-8} M_{\odot}/year, 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 82.13deg. 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<0.5 days. 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.

Figures (11)

• Figure 1: O–C variations of EZ Oct. The diagram is based on a total of 946 minima, including those collected from VarAstro and ASAS-SN, as well as minima extracted from various TESS sectors and our ground-based observations. The linear and parabolic fits are shown on the data, and in the lower panels, their corresponding residuals are displayed.
• Figure 2: Sum of the squared residuals as a function of the mass ratio.
• Figure 3: Phase–Flux observational light curves for five filters: $B$ (blue), $V$ (green), $R$ (red), $I$ (orange), and TESS (purple), arranged from top to bottom with offsets applied for clarity. Horizontal dashed lines indicate the higher of the two maxima for each light curve, highlighting unequal maxima.
• Figure 4: MCMC-derived corner plots for the EZ Oct system.
• Figure 5: Observed (colored dots) and modeled (solid lines) light curves for the systems across $BVR_cI_c$ and TESS filters, arranged from top to bottom.