A Detailed Study of the Physical and Orbital Characteristics, and Eclipse Timing Variations of the Post Common Envelope Binary DD CrB

TL;DR

This work revisits the HW Vir–type post-common-envelope binary DD CrB by integrating new multi-color photometry, extensive RV measurements, and high-precision TESS eclipse timings. A Roemer delay analysis constrains the mass ratio to $q = $0.299 \,±\,0.009$, which, together with joint light-curve–RV modeling via phoebe v2.17, yields a consistent set of stellar and orbital parameters (e.g., $M_1 \,≈ \,0.417 \ M_\\odot$, $M_2 \,≈ \,0.127 \ M_\\odot$, $a \,≈ \,1.019 \ R_\\odot$, $i \,≈ \,86.5^\\circ$). The eclipse timing data reveal a statistically significant 13–14 year modulation best described by a Keplerian light-time effect from a companion with $m_3 \\\sin i \,≈ \,1.30 \,M_{\\mathrm{Jup}}$ and $e_3 \\\approx \,0.46$, while energetic arguments rule out magnetic activity mechanisms. The results strengthen the case for a circumbinary planet around a PCEB and showcase the power of combining multi-instrument datasets with modern binary-star modeling to resolve ETVs and refine fundamental parameters. Continued timing observations are urged to better map the outer orbit and search for additional companions."

Abstract

We present an in-depth analysis of the eclipsing binary DD CrB, composed of a B-type subdwarf primary and an M-type main-sequence secondary, with the main goal of investigating its eclipse timing variations (ETVs). Our new multi-color photometric observations, radial velocity measurements, and precise eclipse timings from TESS allow us to constrain the system parameters. The Romer delay between primary and secondary minima yields a mass ratio of $q = 0.299 \pm 0.009$, enabling robust simultaneous modeling of the light and radial velocity curves with {\sc phoebe} 2.17. By fixing the albedo of the secondary to its maximum physically plausible value (A$_2 = 1.0$), despite the degeneracy between albedo, surface temperature, and radius, we obtained a satisfactory fit, resulting in a significantly lower temperature ($T_2 \sim 2360$ K) and a radius ($R_2 \sim 0.16$ R$_\odot$) in agreement with literature values. Using the total mass of the components and the orbital size derived from this modeling, we interpret the ETVs and find them best explained by a Jupiter-mass tertiary companion on a $\sim13$-year orbit in all competing models, while the eccentric (e $\sim0.46$) models perform better in terms of fit statistics.

Figures (11)

• Figure 1: TESS short-cadence observations of DD CrB spanning 2020--2022. The zoomed-in region highlights a segment of the time-series light curve from Sector 24.
• Figure 2: A sample spectrum covering 3810-5040 Å region acquired with the 1.22 m Asiago Telescope and B&C spectrograph attached on its focal plane.
• Figure 3: The radial velocity data we acquired in the Asiago Observatory (black data points with error bars) the best fit we achieved with the rvfit code (upper panel), and the residuals from the fit (lower panel).
• Figure 4: Light curves of DD CrB in SDSS-ugriz (data points in different colors as given in the legend) and the synthetic light curves (with the same colors) for the model achieved using phoebe by simultaneously modelling all the light curves in different passbands with a fixed value of the albedo for both companions (A$_1$ = A$_2 = 1.0$) (upper panel) , and the residuals from the models (lower panel).
• Figure 5: Radial velocities of DD CrB obtained with different spectrographs (data points) and their model achieved using phoebe. Semi-amplitude of the radial velocity curve is found out to be K$_{1} = 73.40^{0.76}_{0.77}$ km/s from the phoebe model.