  We have found that the 2+2 quadruple star system BU CMi is currently the most
compact quadruple system known, with an extremely short outer period of only
121 days. The previous record holder was TIC 219006972 (Kostov et al. 2023),
with a period of 168 days. The quadruple nature of BU CMi was established by
Volkov et al. (2021), but they misidentified the outer period as 6.6 years. BU
CMi contains two eclipsing binaries (EBs), each with a period near 3 days, and
a substantial eccentricity of about 0.22. All four stars are within about 0.1
solar mass of 2.4 solar masses. Both binaries exhibit dynamically driven
apsidal motion with fairly short apsidal periods of about 30 years, thanks to
the short outer orbital period. The outer period of 121 days is found both from
the dynamical perturbations, with this period imprinted on the eclipse timing
variations (ETV) curve of each EB by the other binary, and by modeling the
complex line profiles in a collection of spectra. We find that the three
orbital planes are all mutually aligned to within 1 degree, but the overall
system has an inclination angle near 83.5 degrees. We utilize a complex
spectro-photodynamical analysis to compute and tabulate all the interesting
stellar and orbital parameters of the system. Finally, we also find an
unexpected dynamical perturbation on a timescale of several years whose origin
we explore. This latter effect was misinterpreted by Volkov et al. (2021) and
led them to conclude that the outer period was 6.6 years rather than the 121
days that we establish here.
