Discovery of a highly eccentric, chromospherically active binary: ASASSN-V J192114.84+624950.8

As part of an All-Sky Automated Survey for SuperNovae (ASAS-SN) search for sources with large flux decrements, we discovered a transient where the quiescent, stellar source ASASSN-V J192114.84+624950.8 rapidly decreased in flux by $\sim 55{{\ \rm per\ cent}}$ (~0.9 mag) in the g band. The Transiting Exoplanet Survey Satellite light curve revealed that the source is a highly eccentric, eclipsing binary. Fits to the light curve using PHOEBE find the binary orbit to have e = 0.79, P_orb = 18.462 d, and i = 88.6°, and the ratios of the stellar radii and temperatures to be R₂/R₁ = 0.71 and T_e,2/T_e,1 = 0.82. Both stars are chromospherically active, allowing us to determine their rotational periods of P₁ = 1.52 d and P₂ = 1.79 d, respectively. A Large Binocular Telescope/Multi-Object Double Spectrograph spectrum shows that the primary is a late-G- or early-K-type dwarf. Fits to the spectral energy distribution show that the luminosities and temperatures of the two stars are L₁ = 0.48 L_⊙, $T_1= 5050\, \mathrm{K}$, L₂ = 0.12 L_⊙, and $T_{2} = 4190\, \mathrm{K}$. We conclude that ASASSN-V J192114.84+624950.8 consists of two chromospherically active, rotational variable stars in a highly elliptical eclipsing orbit.