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Recent Kepler photometry has revealed that about half of white dwarfs (WDs)have periodic, low-level (~ 1e-4 - 1e-3), optical variations. Hubble SpaceTelescope (HST) ultraviolet spectroscopy has shown that up to about one half ofWDs are actively accreting rocky planetary debris, as evidenced by the presenceof photospheric metal absorption lines. We have obtained HST ultravioletspectra of seven WDs that have been monitored for periodic variations, to testthe hypothesis that these two phenomena are causally connected, i.e. that theoptical periodic modulation is caused by WD rotation coupled with aninhomogeneous surface distribution of accreted metals. We detect photosphericmetals in four out of the seven WDs. However, we find no significantcorrespondence between the existence of optical periodic variability and thedetection of photospheric ultraviolet absorption lines. Thus the nullhypothesis stands, that the two phenomena are not directly related. Some othersource of WD surface inhomogeneity, perhaps related to magnetic field strength,combined with the WD rotation, or alternatively effects due to close binarycompanions, may be behind the observed optical modulation. We report themarginal detection of molecular hydrogen in WD J1949+4734, only the fourthknown WD with detected H_2 lines. We also re-classify J1926+4219 as acarbon-rich He-sdO subdwarf.

Periodic optical variability and debris accretion in white dwarfs: a test for a causal connection