WD 1145+017 is currently the only white dwarf known to exhibit periodic transits of planetary debris as well as absorption lines from circumstellar gas .
We present the first simultaneous fast optical spectrophotometry and broad-band photometry of the system , obtained with the Gran Telescopio Canarias ( GTC ) and the Liverpool Telescope ( LT ) , respectively .
The observations spanned 5.5 h , somewhat longer than the 4.5 -h orbital period of the debris .
Dividing the GTC spectrophotometry into five wavelength bands reveals no significant colour differences , confirming grey transits in the optical .
We argue that absorption by an optically thick structure is a plausible alternative explanation for the achromatic nature of the transits that can allow the presence of small-sized ( \sim \mu m ) particles .
The longest ( 87 min ) and deepest ( 50 per cent attenuation ) transit recorded in our data exhibits a complex structure around minimum light that can be well modelled by multiple overlapping dust clouds .
The strongest circumstellar absorption line , Fe ii \lambda 5169 , significantly weakens during this transit , with its equivalent width reducing from a mean out-of-transit value of 2 Å to 1 Å in-transit , supporting spatial correlation between the circumstellar gas and dust .
Finally , we made use of the Gaia Data Release 2 and archival photometry to determine the white dwarf parameters .
Adopting a helium-dominated atmosphere containing traces of hydrogen and metals , and a reddening E ( B - V ) = 0.01 we find T _ { \mathrm { eff } } = 15 020 \pm 520 K , \log g = 8.07 \pm 0.07 , corresponding to M _ { \mathrm { WD } } = 0.63 \pm 0.05 \mbox { $ \mathrm { M } _ { \odot } $ } and a cooling age of 224 \pm 30 Myr .