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We present high-speed, multicolour photometry of the faint, eclipsingcataclysmic variable (CV) SDSS J105754.25+275947.5. The light from this systemis dominated by the white dwarf. Nonetheless, averaging many eclipses revealsadditional features from the eclipse of the bright spot. This enables thefitting of a parameterised eclipse model to these average light curves,allowing the precise measurement of system parameters. We find a mass ratio ofq = 0.0546 $\pm$ 0.0020 and inclination i = 85.74 $\pm$ 0.21$^{\circ}$. Thewhite dwarf and donor masses were found to be M$_{\mathrm{w}}$ = 0.800 $\pm$0.015 M$_{\odot}$ and M$_{\mathrm{d}}$ = 0.0436 $\pm$ 0.0020 M$_{\odot}$,respectively. A temperature T$_{\mathrm{w}}$ = 13300 $\pm$ 1100 K and distanced = 367 $\pm$ 26 pc of the white dwarf were estimated through fitting modelatmosphere predictions to multicolour fluxes. The mass of the white dwarf inSDSS 105754.25+275947.5 is close to the average for CV white dwarfs, while thedonor has the lowest mass yet measured in an eclipsing CV. A low-mass donor andan orbital period (90.44 min) significantly longer than the period minimumstrongly suggest that this is a bona fide period-bounce system, althoughformation from a white dwarf/brown dwarf binary cannot be ruled out. Very fewperiod-minimum/period-bounce systems with precise system parameters arecurrently known, and as a consequence the evolution of CVs in this regime isnot yet fully understood.

SDSS J105754.25+275947.5: a period-bounce eclipsing cataclysmic variable with the lowest-mass donor yet measured