ASASSN-21co: A Detached Eclipsing Binary with an 11.9 yr Period

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