We report the discovery of a wide comoving substellar companion to the nearby ( D = 67.5 \pm 1.1 pc ) A3V star \zeta Delphini based on imaging and follow-up spectroscopic observations obtained during the course of our Volume-limited A-Star ( VAST ) multiplicity survey . \zeta Del was observed over a five-year baseline with adaptive optics , revealing the presence of a previously-unresolved companion with a proper motion consistent with that of the A-type primary .
The age of the \zeta Del system was estimated as 525 \pm 125 Myr based on the position of the primary on the colour-magnitude and temperature-luminosity diagrams .
Using intermediate-resolution near-infrared spectroscopy , the spectrum of \zeta Del B is shown to be consistent with a mid-L dwarf ( L 5 \pm 2 ) , at a temperature of 1650 \pm 200 K. Combining the measured near-infrared magnitude of \zeta Del B with the estimated temperature leads to a model-dependent mass estimate of 50 \pm 15 M _ { Jup } , corresponding to a mass ratio of q = 0.019 \pm 0.006 .
At a projected separation of 910 \pm 14 au , \zeta Del B is among the most widely-separated and extreme-mass ratio substellar companions to a main-sequence star resolved to-date , providing a rare empirical constraint of the formation of low-mass ratio companions at extremely wide separations .