2000A&A...354L..49B

Low-state spectrophotometry of the short-period polar EF Eridani (P_orb=81min) found the system at V=18.0 with no trace of the companion (Wheatley and Ramsay, 1998). We show that the lack of such spectral features implies that the companion to the white dwarf in EF Eri has a spectral type later than M9 and is either a transition object at the brink of hydrogen burning or a brown dwarf. The optical low state spectrum indicates a temperature of the white dwarf of T_eff=9500±500K. This is one of the coldest white dwarfs in cataclysmic variables, implying a cooling age t_ cool_>10⁹yrs or accretional heating at a rate as given by gravitational radiation. The large age of the system excludes a warm brown dwarf as companion. EF Eri has either just passed through the period minimum of cataclysmic variable stars or has started mass transfer from an old brown dwarf secondary.