THEKEPLERLIGHT CURVE OF THE UNIQUE DA WHITE DWARF BOKS 53856

The faint ( g = 16.9) hot white dwarf BOKS 53856 was observed by the Kepler Mission in short cadence mode during mid-2009. Analysis of these observations reveals a highly stable modulation with a period of 6.1375 hr and a 2.46% half-amplitude. The folded light curve has an unusual shape that is difficult to explain in terms of a binary system containing an unseen companion more luminous than an L0 brown dwarf. Optical spectra of BOKS 53856 show a T eff = 34,000 K, log g = 8.0 DA white dwarf. There are few, if any, known white dwarfs in this temperature range exhibiting photometric variations similar to those we describe. A magnetic spin-modulated white dwarf model can in principle explain the light curve, an interpretation supported by spectral observations of the Hα line showing evidence of Zeeman splitting.