  We report the discovery of J1953-1019, the first resolved triple white dwarf
system. The triplet consists of an inner white dwarf binary and a wider
companion. Using Gaia DR2 photometry and astrometry combined with our follow-up
spectroscopy, we derive effective temperatures, surface gravities, masses and
cooling ages of the three components. All three white dwarfs have pure-hydrogen
(DA) atmospheres, masses of 0.60-0.63 Msun and cooling ages of 40-290 Myr. We
adopt eight initial-to-final mass relations to estimate the main sequence
progenitor masses (which we find to be similar for the three components,
1.6-2.6 Msun) and lifetimes. The differences between the derived cooling times
and main sequence lifetimes agree for most of the adopted initial-to-final mass
relations, hence the three white dwarfs in J1953-1019 are consistent with
coeval evolution. Furthermore, we calculate the projected orbital separations
of the inner white dwarf binary (303.25 +- 0.01 au) and of the centre of mass
of the inner binary and the outer companion (6398.97 +- 0.09 au). From these
values, and taking into account a wide range of possible configurations for the
triplet to be currently dynamically stable, we analyse the future evolution of
the system. We find that a collision between the two inner white dwarfs due to
Lidov-Kozai oscillations is unlikely, though if it occurs it could result in a
sub-Chandrasekhar Type Ia supernova explosion.

Solar and Stellar Astrophysics