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We analyse spectroscopic and photometric observations of the eclipsing polar BS Tri. The polar's light-curve-shape variations can be interpreted by changing contributions of the accretion stream to the integral radiation of the system. Based on the radial-velocity curves of the irradiated part of the secondary, we refine the masses of the system components, M 1 = 0.60 ± 0.04  M ⊙ and M 2 ≈ 0.12  M ⊙, and the orbital inclination, i  = 85 ± 0.5°. The polar's spectra reveal cyclotron harmonics forming in an accretion spot with a magnetic field strength of B  = 22.7 ± 0.4 MG and an average temperature of T ∼ 10 keV. In addition to the cyclotron harmonics, the BS Tri spectra contain Zeeman components of H α line, which are probably formed in the cool halo near the accretion spot. The orientation of the magnetic dipole and the coordinates of the accretion spot are estimated by modelling the light curves of the polar. We show that for a satisfactory description of the BS Tri light curves we have to take into account the variability of the spot's optical depth along the line of sight. Doppler maps of BS Tri show a part of the accretion stream with a trajectory close to ballistic near the Lagrange point L1, and another part of the stream moving along the magnetic field lines. The estimate of the stagnation region position found from the Doppler tomograms is consistent with the photometric estimates of the accretion spot position. [ABSTRACT FROM AUTHOR]