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The star 1SWASP J024743.37-251549.2 was recently discovered to be a binarystar in which an A-type dwarf star eclipses the remnant of a disrupted redgiant star (WASP0247-25B). The remnant is in a rarely-observed state evolvingto higher effective temperatures at nearly constant luminosity prior tobecoming 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 17eclipsing binary stars with orbital periods P=0.7 to 2.2 days with similarlightcurves to 1SWASP J024743.37-251549.2. The only star in this grouppreviously identified as a variable star is the brightest one, EL CVn, which weadopt as the prototype for this class of eclipsing binary star. Thecharacteristic lightcurves of EL CVn-type stars show a total eclipse by anA-type dwarf star of a smaller, hotter star and a secondary eclipse ofcomparable depth to the primary eclipse. We have used new spectroscopicobservations for 6 of these systems to confirm that the companions to theA-type stars in these binaries have very low masses (approximately 0.2 solarmasses). This includes the companion to EL CVn which was not previously knownto be a pre-He-WD. EL CVn-type binary star systems will enable us to study theformation of very low-mass white dwarfs in great detail, particularly in thosecases where the pre-He-WD star shows non-radial pulsations similar to thoserecently discovered in WASP0247-25B.

EL CVn-type binaries - Discovery of 17 helium white dwarf precursors in bright eclipsing binary star systems