Optical Study of the Polar BM CrB in a Low State

This paper presents a spectral and photometric study of the poorly studied polar BM CrB. Three states brightness signs transition from one-pole to two-pole accretion mode were found by an analysis ZTF data. It is shown that low state high changes longitude main spot (by $\approx 17^{\circ}$) increases its elongation 10^{\circ}$). The spectra contain Zeeman absorptions H$\alpha$ line which are formed at magnetic field strength $15.5\pm1$ MG. These likely produced cold halo extending {^1/_4}$ white dwarf radius. Modeling behavior emission shows source part stream near Lagrangian point L$_1$, periodically eclipsed donor star. exhibit cyclotron component in spot. Its modeling simple model gives constraints on $B=15-40$ MG temperature $T_e\gtrsim15$ keV.