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The first transiting planetesimal orbiting a white dwarf was recentlydetected in K2 data of WD1145+017 and has been followed up intensively. Themultiple, long, and variable transits suggest the transiting objects are dustclouds, probably produced by a disintegrating asteroid. In addition, the systemcontains circumstellar gas, evident by broad absorption lines, mostly in theu'-band, and a dust disc, indicated by an infrared excess. Here we present thefirst detection of a change in colour of WD1145+017 during transits, usingsimultaneous multi-band fast-photometry ULTRACAM measurements over theu'g'r'i'-bands. The observations reveal what appears to be 'bluing' duringtransits; transits are deeper in the redder bands, with a u'-r' colourdifference of up to ~-0.05 mag. We explore various possible explanations forthe bluing. 'Spectral' photometry obtained by integrating over bandpasses inthe spectroscopic data in- and out-of-transit, compared to the photometricdata, shows that the observed colour difference is most likely the result ofreduced circumstellar absorption in the spectrum during transits. Thisindicates that the transiting objects and the gas share the same line-of-sight,and that the gas covers the white dwarf only partially, as would be expected ifthe gas, the transiting debris, and the dust emitting the infrared excess, arepart of the same general disc structure (although possibly at different radii).In addition, we present the results of a week-long monitoring campaign of thesystem.

Once in a blue moon: detection of 'bluing' during debris transits in the white dwarf WD1145+017