ZTF long-term (top left) and phase-folded (top right) light curves in g and r filters and the DBSP optical spectrum of 2CXO J182117.2–131405 (bottom). The gray lines represent the Keck Telluric Line List. The optical spectrum is dominated by broad humps that correspond to cyclotron harmonics, and no significant Hβ and He II 4686 Å emission lines are detected. A combination of the cyclotron wavelengths seen in the spectrum and the large beaming amplitude constrains the magnetic field to B ≈ 54 − 63 MG. Optical and X-ray properties suggest that the object is a low accretion rate polar (see Sect. 5.1.4 for more details).