  We report the first radial velocity spectroscopic study of the eclipsing
period bouncer SDSS J105754.25+275947.5. Together with eclipse light curve
modeling, we redetermined the system parameters and studied the accretion disk
structure. We confirm that the system contains a white dwarf with
$M_{\mathrm{WD}}=0.83(3) M_\odot$ and an effective temperature of 11,500(400)K.
The mass of the secondary is $M_2 = 0.056 M_\odot$ with an effective
temperature of T$_2$=2,100K or below. The system inclination is $i=84.3(6)$.
The data is in good agreement with our determination of $K_1$ = 33(4) km
s$^{-1}$. We estimate the mass transfer rate as $\dot{M}=$1.9(2)$\times
10^{-11} M_\odot 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 disk is low and
originates from the outer part of the disk. The Balmer emission lines are
formed in a plasma with $\log$ $N_0$ = 12.7 [cm$^{-1}$] and a kinetic
temperature of T$\sim$10,000K. The size of the disk, where the emission lines
are formed, expands up to $R_\mathrm{d,out}=0.29 R_\odot$. The inner part of
the emission line forming region goes down to $R_\mathrm{d,in}\approx 2
R_\mathrm{WD}$ . The Doppler tomography and trailed spectra show the presence
of a hot spot and a clumpy structure in the disk, with variable intensity along
the disk position angle. There is an extended region at the side opposite the
hot spot with two bright clumps caused more probably by non-Keplerian motion
there.
