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In an XMM-Newton observation of the binary SDSS J121209.31+013627.7,consisting of a white dwarf and an L dwarf, we detect X-ray orbital modulationas proof of accretion from the substellar companion onto the magnetic whitedwarf. We constrain the system geometry (inclination as well as magnetic andpole-cap angle) through modelling of the X-ray light curve, and we derive amass accretion rate of 3.2 10^(-14) M_sun/yr from the X-ray luminosity (~ 310^(29) erg/s). From X-ray studies of L dwarfs, a possible wind driven from ahypothesized corona on the substellar donor is orders of magnitude too weak toexplain the observed accretion rate, while the radius of the L dwarf iscomparable to its Roche lobe (0.1 R_sun), making Roche-lobe overflow the likelyaccretion mechanism in this system.

X-ray orbital modulation of a white dwarf accreting from an L dwarf: The system SDSS J121209.31+013627.7