ASASSN-21co: A detached eclipsing binary with an 11.9 year period

TL;DR

This paper models the long-period detached eclipsing binary ASASSN-21co, revealing it consists of two M giants with an 11.9-year orbit, using multi-band photometry and spectral analysis to characterize its properties.

Contribution

The study provides the first detailed modeling of ASASSN-21co's orbital and stellar parameters, highlighting its unique long period and giant star components.

Findings

Orbital period of 11.9 years determined from eclipse observations.

System consists of two M giant stars with a radius ratio of ~0.61.

Eclipse shows asymmetry likely due to rotational variability.

Abstract

We use ASAS V-band and ASAS-SN g-band observations to model the long-period detached eclipsing binary ASASSN-21co. ASAS observations show an eclipse of depth V ~ 0.6 mag in April of 2009. ASAS-SN g-band observations from March of 2021 show an eclipse of similar duration and depth, suggesting an orbital period of 11.9 years. We combine the g-band observations with additional BVRI photometry taken during the eclipse to model the eclipse using PHOEBE. We find that the system is best described by two M giants with a ratio of secondary radius to primary radius of ~0.61. Optical spectra taken during the eclipse are consistent with at least one component of the binary being an M giant, and we find no temporal changes in the spectral features. The eclipse itself is asymmetric, showing an increase in brightness near mid-eclipse, likely due to rotational variability that is too low amplitude to be observed out-of-eclipse.