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

Abstract

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