2009MNRAS.394..978P

We identify SDSS011009.09+132616.1, SDSS030308.35+005444.1, SDSS143547.87+ 373338.5 and SDSS154846.00+405728.8 as four eclipsing white dwarf plus main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS), and report on follow-up observations of these systems. SDSS0110+1326, SDSS1435+3733 and SDSS1548+4057 contain DA white dwarfs, while SDSS0303+0054 contains a cool DC white dwarf. Orbital periods and ephemerides have been established from multiseason photometry. SDSS1435+3733, with P_orb= 3h has the shortest orbital period of all known eclipsing WDMS binaries. As for the other systems, SDSS0110+1326 has P_orb= 8h, SDSS0303+0054 has P_orb= 3.2h and SDSS1548+4057 has P_orb= 4.4h. Time-resolved spectroscopic observations have been obtained and the Hα and Caiiλλ8498.02, 8542.09, 8662.14 triplet emission lines, as well as the Naiλλ8183.27, 8194.81 absorption doublet were used to measure the radial velocities of the secondary stars in all four systems. A spectral decomposition/fitting technique was then employed to isolate the contribution of each of the components to the total spectrum, and to determine the white dwarf effective temperatures and surface gravities, as well as the spectral types of the companion stars. We used a light-curve modelling code for close binary systems to fit the eclipse profiles and the ellipsoidal modulation/reflection effect in the light curves, to further constrain the masses and radii of the components in all systems. All three DA white dwarfs have masses of M_WD∼ 0.4-0.6M_☉, in line with the expectations from close binary evolution. The DC white dwarf in SDSS0303+0054 has a mass of M_WD≳ 0.85M_☉, making it unusually massive for a post-common-envelope system. The companion stars in all four systems are M dwarfs of spectral type M4 and later. Our new additions raise the number of known eclipsing WDMS binaries to 14, and we find that the average white dwarf mass in this sample is <M_WD> =0.57±0.16M_☉, only slightly lower than the average mass of single white dwarfs. The majority of all eclipsing WDMS binaries contain low-mass (<0.6M_☉) secondary stars, and will eventually provide valuable observational input for the calibration of the mass-radius relations of low-mass main-sequence stars and of white dwarfs.