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We use ASAS V-band and ASAS-SN g-band observations to model the long-perioddetached eclipsing binary ASASSN-21co. ASAS observations show an eclipse ofdepth V ~ 0.6 mag in April of 2009. ASAS-SN g-band observations from March of2021 show an eclipse of similar duration and depth, suggesting an orbitalperiod of 11.9 years. We combine the g-band observations with additional BVRIphotometry taken during the eclipse to model the eclipse using PHOEBE. We findthat the system is best described by two M giants with a ratio of secondaryradius to primary radius of ~0.61. Optical spectra taken during the eclipse areconsistent with at least one component of the binary being an M giant, and wefind no temporal changes in the spectral features. The eclipse itself isasymmetric, showing an increase in brightness near mid-eclipse, likely due torotational variability that is too low amplitude to be observed out-of-eclipse.

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