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 similar to 55 per cent (similar to 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 degrees, and the ratios of the stellar radii and temperatures to be R-2/R-1 = 0.71 and T-e,T-2/T-e,T-1 = 0.82. Both stars are chromospherically active, allowing us to determine their rotational periods of P-1 = 1.52 d and P-2 = 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 L-1 = 0.48 L-circle dot, T-1= 5050 K, L-2 = 0.12 L-circle dot, and T-2 = 4190 K. We conclude that ASASSN-V J192114.84+624950.8 consists of two chromospherically active, rotational variable stars in a highly elliptical eclipsing orbit.