2023MNRAS.526.5110E

We report the first radial velocity spectroscopic study of the eclipsing period bouncer SDSS J105754.25+275947.5. Together with eclipse light-curve modelling, we re-determined the system parameters and studied the accretion disc structure. We confirm that the system contains a white dwarf with M_WD = 0.83(3) M_⊙ and an effective temperature of 11 500(400) K. The mass of the secondary is M₂ = 0.056 M_⊙ with an effective temperature of T₂ = 2100 K or below. The system inclination is i = 84.3(6)°. The data are in good agreement with our determination of K₁ = 33(4) km s^–1. We estimate the mass transfer rate as $\dot{M}=1.9(2)\times 10^{-11}$ M_⊙ yr ^–1. Based on an analysis of the SDSS and OSIRIS spectra, we conclude that the optical continuum is formed predominantly by the radiation from the white dwarf. The contribution of the accretion disc is low and originates from the outer part of the disc. The Balmer emission lines are formed in a plasma with log N₀ = 12.7 (cm^–1) and a kinetic temperature of T ∼ 10 000 K. The size of the disc, where the emission lines are formed, expands up to R_d,out = 0.29 R_⊙. The inner part of the emission line forming region goes down to R_d,in ≈ 2R_WD. The Doppler tomography and trailed spectra show the presence of a hotspot and a clumpy structure in the disc, with variable intensity along the disc position angle. There is an extended region at the side opposite the hotspot with two bright clumps caused more probably by non-Keplerian motion there.