The Physical Nature of Subdwarf A Stars: White Dwarf Impostors
  We address the physical nature of subdwarf A-type (sdA) stars and their
possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes
of objects are confused in low-resolution spectroscopy. However, colors and
proper motions indicate that sdA stars are cooler and more luminous, and thus
larger in radius, than published ELM WDs. We demonstrate that surface gravities
derived from pure hydrogen models suffer a systematic ~1 dex error for sdA
stars, likely explained by metal line blanketing below 9000 K. A detailed study
of five eclipsing binaries with radial velocity orbital solutions and infrared
excess establishes that these sdA stars are metal-poor ~1.2 Msun main sequence
stars with ~0.8 Msun companions. While WDs must exist at sdA temperatures, only
~1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the
majority of sdA stars are metal-poor A-F type stars in the halo, and that
recently discovered pulsating ELM WD-like stars with no obvious radial velocity
variations may be SX Phe variables, not pulsating WDs.
