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-circle dot. By fitting models to the spectrum of this star around the H gamma line we find that it has an effective temperature T-eff,T- 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-circle dot. The effective temperature of this pre-He-WD is approximately 9200 K. The UL-TRACAM photometry of WASP 1628+10 shows variability at several frequencies around 40 cycles d(-1), which is typical for delta 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 delta 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.