  The physical nature of the sdA stars---cool hydrogen-rich objects with
spectroscopic surface gravities intermediate between main sequence and
canonical mass white dwarfs---has been elusive since they were found in Sloan
Digital Sky Survey Data Release 12 spectra. The population is likely dominated
by metal-poor A/F stars in the halo with overestimated surface gravities, with
a small contribution of extremely low-mass white dwarfs and their precursors,
i.e., ELMs and pre-ELMs. In this work, we seek to identify (pre-)ELMs with
radii smaller than is possible for main sequence stars, allowing even for very
low metallicity. We analyse 3891 sdAs previously identified in the Sloan
Digital Sky Survey using Gaia DR2 data. Our Monte Carlo analysis supports that
90 of these are inconsistent with the main sequence. 37 lie close to or within
the canonical white dwarf cooling sequence, while the remaining 53 lie between
the canonical white dwarfs and main sequence, which we interpret as likely
(pre-)ELMs given their spectral class. Of these, 30 pass more conservative
criteria that allow for higher systematic uncertainties on the parallax, as
well as an approximate treatment of extinction. Our identifications increase
the number of known (pre-)ELMs by up to 50 per cent, demonstrating how Gaia
astrometry can reveal members of the compact (pre-)ELM subpopulation of the sdA
spectral class.
