Measuring the rotational periods of isolated magnetic white dwarfs

We present time-series photometry of 30 isolated magnetic white dwarfs, surveyed with the Jacobus Kapteyn Telescope between 2002 August and 2003 May. We find that 9 were untestable due to varying comparison stars, but of the remaining 21, 5 (24%) are variable with reliably derived periods, while a further 9 (43%) are seen to vary during our study, but we were unable to derive the period. We interpret the variability to be the result of rotation of the objects. We find no correlation between rotation period and mass, temperature, magnetic field, or age. We have found variability in 9 targets with low magnetic field strengths and temperatures low enough for partially convective atmospheres, which we highlight as candidates for polarimetry to search for starspots. Most interestingly, we have found variability in one target, PG1658+441, which has a fully radiative atmosphere in which conventional starspots cannot form, but a magnetic field strength that is too low to cause magnetic dichroism. The source of variability in this target remains a mystery.