  We report the discovery of a third white dwarf hosting a gaseous debris disc,
SDSSJ084539.17+225728.0. The typical double-peaked CaII 8498,8542,8662A
emission lines can be modelled in terms of a Keplerian gas disc with a radial
extent from ~0.5Rsun to ~1.0. The effective temperature of SDSS0845+2257,
Teff=18600+-500K, is comparable to the two other white dwarfs with gaseous
discs, SDSS1043+0855 and SDSS1228+1040, and hence substantially hotter than the
bulk of white dwarfs where dusty debris discs were identified through the
presence of infrared excess flux. This may suggest that the conditions to
produce emission lines from debris discs in the optical wavelength range are
only met for a relatively narrow range in Teff. The observed asymmetry in the
line profiles indicates a substantial eccentricity in the disc. Two spectra
obtained four years apart reveal a significant change in the shapes and
equivalent widths of the line profiles, implying that the circumstellar disc
evolves on relatively short time scales. In contrast to SDSS1043+0855 and
SDSS1228+1040, SDSS0845+2257 has a helium-dominated atmosphere. We detect
photospheric absorption lines of He, Ca, Mg, and Si in the Sloan Digital Sky
Survey spectrum, and hence classify SDSS0845+2257 as DBZ white dwarf. The
abundances for the three metals determined from model atmosphere fits are
Ca/He~1.3x10^-7, Mg/He~6.0x10^-6, and Si/He~8.0x10^-6. From the non-detection
of Halpha we derive H/He<3x10^-5, which implies that the hydrogen-to-metal
abundance ratio of the circumstellar material is >~1000 times lower than in the
Sun. This lends strong support to the hypothesis that the gaseous and dusty
debris discs found around roughly a dozen white dwarfs originate from the
disruption of rocky planetary material.
