  Gaia DR2 provides a unique all-sky catalogue of 550'737 variable stars, of
which 151'761 are long-period variable (LPV) candidates with G variability
amplitudes larger than 0.2 mag (5-95% quantile range). About one-fifth of the
LPV candidates are Mira candidates, the majority of the rest are semi-regular
variable candidates. For each source, G, BP , and RP photometric time-series
are published, together with some LPV-specific attributes for the subset of
89'617 candidates with periods in G longer than 60 days. We describe this first
Gaia catalogue of LPV candidates, and present various validation checks.
Various samples of LPVs were used to validate the catalogue: a sample of
well-studied very bright LPVs with light curves from the AAVSO that are partly
contemporaneous with Gaia light curves, a sample of Gaia LPV candidates with
good parallaxes, the ASAS_SN catalogue of LPVs, and the OGLE catalogues of LPVs
towards the Magellanic Clouds and the Galactic bulge. The analyses of these
samples show a good agreement between Gaia DR2 and literature periods. The same
is globally true for bolometric corrections of M-type stars. The main
contaminant of our DR2 catalogue comes from young stellar objects (YSOs) in the
solar vicinity (within ~1 kpc), although their number in the whole catalogue is
only at the percent level. A cautionary note is provided about
parallax-dependent LPV attributes published in the catalogue. This first Gaia
catalogue of LPVs approximately doubles the number of known LPVs with
amplitudes larger than 0.2 mag, despite the conservative candidate selection
criteria that prioritise low contamination over high completeness, and despite
the limited DR2 time coverage compared to the long periods characteristic of
LPVs. It also contains a small set of YSO candidates, which offers the
serendipitous opportunity to study these objects at an early stage of the Gaia
data releases.
