Using TESS observations, we independently discovered that BU Canis Minoris (BU CMi, TIC 271204362), previously thought to be an eclipsing binary with a 2.94d period, is actually a quadruply-eclipsing four-star system (2+2). Its two component binaries have periods of 2.94d and 3.26d. We found the outer orbit to have a period of ~120 days, the shortest known among such systems. Moreover, BU CMi exhibits strongly driven apsidal precession, which we discovered through analyzing eclipse times and durations from TESS sectors 7 and 34, as well as KELT and MASCARA data. Both binaries’ apsides are found to precess with a period of approximately 25 years. Our analyses of 39 high-resolution spectra of this star suggest that its four components are early-type B/A stars with Teff ~11,000 K; however, disentangling the four individual radial velocities from the spectral lines has proven difficult in this system. So, we both iteratively and jointly analyzed the spectra and orbital motion. This helped confirm the outer period and estimate its semimajor axis. We also used a photodynamics code to analyze the eclipses in order to independently evaluate all three orbits (of binaries A, B, and the outer one). Future work will involve further ground-based photometry to refine its system parameters, after the sun moves away from it (in a few months).

{"references": ["Powell, Brian et al. (2021). TIC 168789840: A Sextuply-Eclipsing Sextuple Star System."]}