Binaries discovered by the MUCHFUSS project

FBS 0117+396: An sdB+dM binary with a pulsating primary^⋆

¹ Instituut voor Sterrenkunde, K. U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: roy@ster.kuleuven.be
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Dr. Karl-Remeis-Observatory & ECAP, Astronomical Institute, F.-A. U. Erlangen-Nürnberg, 96049 Bamberg, Germany
⁴ Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁵ Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, 38711 Breña Baja, Spain
⁶ Department of Astrophysics, IMAPP, Radboud University Nijmegen, 6500 GL Nijmegen, The Netherlands
⁷ Observatorio Astronómico Nacional, Universidad Nacional Autónoma de México, BC 22860 Ensenada, Mexico
⁸ Dept of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
⁹ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
¹⁰ Department of Physics, University of Warwick, Coventry CV4 7AL, UK

Received: 5 June 2013
Accepted: 30 September 2013

Abstract

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims to discover subdwarf-B stars with massive compact companions such as overmassive white dwarfs (M > 1.0 M_⊙), neutron stars or black holes. From the 127 subdwarfs with substantial radial-velocity variations discovered in the initial survey, a number of interesting objects have been selected for extensive follow-up. After an initial photometry run with BUSCA revealed that FBS 0117+396 is photometrically variable both on long and short timescales, we chose it as an auxiliary target during a 6-night multi-color photometry run with Ultracam. Spectroscopy was obtained at a number of observatories in order to determine the binary period and obtain a radial-velocity amplitude. After establishing an orbital period of P  =  0.252 d, and removing the signal associated with the irradiated hemisphere of the M-dwarf companion, we were able to detect ten pulsation periods in the Fourier spectrum of the light curve. Two pulsation modes are found to have short periods of 337 and 379 s, and at least eight modes are found with periods between 45 min and 2.5 h. This establishes that FBS 0117+396 is an sdB+dM reflection binary, in which the primary is a hybrid pulsator, and the first one found with this particular mélange of flavours.