Photometric variability of the unique magnetic white dwarf GD 356

GD 356 is a magnetic white dwarf (B = 13 MG) that uniquely displays weak resolved Zeeman triplets of Halpha and Hbeta in emission. As such, GD 356 may be the only known white dwarf with some kind of chromosphere, although accretion from the interstellar medium or more exotic mechanisms cannot be ruled out. Here, we report the detection of low amplitude (+/- similar to0.2 per cent) near-sinusoidal photometric (V-band) variability in GD 356, with a period of 0.0803 d (similar to115 min). We interpret this as the rotation period of the star. We model the variability with a dark spot (by analogy with star spots) covering 10 per cent of the stellar surface. It seems likely that this spot is also the site of the Zeeman emission, requiring the presence of a temperature inversion. We show that the spot is never totally visible or obscured, and that both polar and equatorial spots produce good fits to the data when viewed at high and low inclination respectively.