HS 2333 + 3927: A NEW sdB BINARY WITH A LARGE REFLECTION EFFECT C.A. KARL 1 , U. HEBER 1 , H. DRECHSEL 1 , R. NAPIWOTZKI 1 , M. ALTMANN 1 , R. ØSTENSEN 2 , S. FOLKES 2 , J.E. SOLHEIM 3 , O. CORDES 4 , B. VOSS 5 and D. KOESTER 5 1 Dr. Remeis-Sternwarte Bamberg, Astronomisches Institut der Universit¨ at Erlangen-N¨ urnberg, Sternwartstraße 7, D-96049 Bamberg, Germany; E-mail: karl@sternwarte.uni-erlangen.de 2 Isaac Newton Group of Telescopes, E-37800 Santa Cruz de La Palma, Canary Islands, Spain; 3 University of Tromsø, Department of Physics, N-9037 Tromsø, Norway; 4 Sternwarte der Universit¨ at Bonn, Auf dem H¨ ugel 71, 53121 Bonn, Germany; 5 Institut f ¨ ur Theoretische Physik und Astrophysik, Universit¨ at Kiel, 24098 Kiel, Germany; Abstract. We report the discovery of a binary, HS 2233+3927, consisting of an sdB star with a faint companion. From its lightcurve the orbital period of 14,844 s, the mass ratio, the inclination, and other system parameters are derived. The companion does not contribute to the optical light of the system except through a strong reflection effect. The semi-amplitude of the radial velocity curve K 1 = 89.6 km/s −1 and a mass function of f (m) = 0.013 M ⊙ are determined. A preliminary spectroscopic analysis of the blue spectra using NLTE model atmospheres results in T eff = 36 500 K, log g = 5.70, and log(n He / n H ) =−2.15. These parameters are typical for sdB stars, the companion is probably an M dwarf. Keywords: stars: binary, stars: subdwarfs 1. Introduction Considerable evidence is accumulating that many sdB stars reside in close binaries (Maxted et al., 2001, and Saffer et al., 2001). Therefore mass transfer should play an important role in the evolution of such binary systems. Detailed investigations of sdB binaries, in particular eclipsing systems, are crucial for determining their masses. However, only three such eclipsing binaries, HW Vir (Menzies and Marang 1986), PG 1336 – 018 (Kilkenny et al., 1998), and HS 0705+6700 (Drechsel et al., 2001) are known up to now, which consist of an sdB star and an optically invisible M dwarf companion. Here we report the discovery of another related system– HS 2333+3927, which is not eclipsing, but otherwise possesses very similar system parameters and configuration. 2. Photometry The first photometric observations of HS 2333+3927 were made in a monitoring run at the Nordic Optical Telescope on 19 Oct., 1999 (Østensen, 2000). Further Astrophysics and Space Science 291: 283–289, 2004. C  2004 Kluwer Academic Publishers. Printed in the Netherlands.