Backyard Worlds: Planet 9 Discovery of an Unusual Low-mass Companion to an M Dwarf at 80 pc

We present the discovery of CWISE J203546.35–493611.0, a peculiar M8 companion to the M4.5 star APMPM J2036−4936 discovered through the citizen science project Backyard Worlds: Planet 9. Given CWISE J203546.35–493611.0's proper motion (μ α , μ δ ) = (−126 ± 22, −478 ± 23) and angular separation of 342 from APMPM 2036−4936, we calculate a chance alignment probability of 1.15 × 10−6. Both stars in this system appear to be underluminous, and the spectrum obtained for CWISE J203546.35–493611.0 shows a triangular H band. Further study of this system is warranted to understand these peculiarities.