WASP 1628+10-an EL CVn-type binary with a very low mass stripped red giant star and multiperiodic pulsations

The star 1SWASP J162842.31+101416.7 (WASP 1628+10) is one of several EL CVn-type stars recently identified using the Wide Angle Search for Planets (WASP) data base, i.e. an eclipsing binary star in which an A-type dwarf star (WASP 1628+10 A) eclipses the remnant of a disrupted red giant star (WASP 1628+10 B). We have measured the masses, radii and luminosities of the stars in WASP 1628+10 using photometry obtained in three bands (u , g , r ) with the ULTRACAM instrument and medium-resolution spectroscopy. The properties of the remnant are well matched by models for stars 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. we confirm that WASP 1628+10 B is a precursor of a helium white dwarf (pre-He-WD). WASP 1628+10 A appears to be a normal A2 V star with a mass of 1.36 ± 0.05 M. By fitting models to the spectrum of this star around the Hγ line we find that it has an effective temperature Teff, A = 7500 ± 200 K and a metallicity [Fe/H] = −0.3 ± 0.3. The mass of WASP 1628+10 B is only 0.135 ± 0.02 M. The effective temperature of this pre-He-WD is approximately 9200 K. The ULTRACAM photometry of WASP 1628+10 shows variability at several frequencies around 40 cycles d−1, which is typical for δ Sct-type pulsations often observed in early A-type stars like WASP 1628+10 A. We also observe frequencies near 114 and 129 cycles d−1, much higher than the frequencies normally seen in δ Sct stars. Additional photometry through the primary eclipse will be required to confirm that these higher frequencies are due to pulsations in WASP 1628+10 B. If confirmed, this would be only the second known example of a pre-He-WD showing high-frequency pulsations.