GALEX and Optical Light Curves of EF Eridanus during a Low State: The Puzzling Source of Ultraviolet Light

Low-state optical photometry of EF Eri during an extended low accretion state combined with GALEX near- and far-UV time-resolved photometry reveals a source of UV flux that is much larger than the underlying 9500 K white dwarf and that is highly modulated on the orbital period. The near-UV and optical light curves can be modeled with a 20,000 K spot, but no spot model can explain both the large-amplitude FUV variations and the SED. The limitations of limb darkening, cyclotron, and magnetic white dwarf models in explaining the observations are discussed.