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

P.F.L. Maxted, S. Bloemen, U. Heber, S. Geier, P.J. Wheatley, T.R. Marsh, E. Breedt, D. Sebastian, G. Faillace, C. Owen, D. Pulley, D. Smith, U. Kolb, C.A. Haswell, J. Southworth, D.R. Anderson, B. Smalley, A. Collier Cameron, L. Hebb, E.K. SimpsonR.G. West, J. Bochinski, R. Busuttil, S. Hadigal

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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 (WASP 0247-25 B). 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-helium white dwarf (pre-He-WD). We have used the photometric database from theWide Angle Search for Planets (WASP) to find 17 eclipsing binary stars with orbital periods P = 0.7-2.2 d with similar light curves 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 light curves 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 six of these systems to confirm that the companions to the A-type stars in these binaries have very low masses (≈0.2M⊙). 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 WASP0247-25 B.
Original languageEnglish
Pages (from-to)1681-1697
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date20 Nov 2013
Publication statusPublished - 11 Jan 2014


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