  The star 1SWASP J024743.37-251549.2 was recently discovered to be a binary
star in which an A-type dwarf star eclipses the remnant of a disrupted red
giant star (WASP0247-25B). The remnant is in a rarely-observed state evolving
to higher effective temperatures at nearly constant luminosity prior to
becoming a very low-mass white dwarf composed almost entirely of helium, i.e.,
it is a pre-He-WD. We have used the WASP photometric database to find 17
eclipsing binary stars with orbital periods P=0.7 to 2.2 days with similar
lightcurves to 1SWASP J024743.37-251549.2. The only star in this group
previously identified as a variable star is the brightest one, EL CVn, which we
adopt as the prototype for this class of eclipsing binary star. The
characteristic lightcurves of EL CVn-type stars show a total eclipse by an
A-type dwarf star of a smaller, hotter star and a secondary eclipse of
comparable depth to the primary eclipse. We have used new spectroscopic
observations for 6 of these systems to confirm that the companions to the
A-type stars in these binaries have very low masses (approximately 0.2 solar
masses). This includes the companion to EL CVn which was not previously known
to be a pre-He-WD. EL CVn-type binary star systems will enable us to study the
formation of very low-mass white dwarfs in great detail, particularly in those
cases where the pre-He-WD star shows non-radial pulsations similar to those
recently discovered in WASP0247-25B.
