The Mass of the sdB Primary of the Binary HS 2333+3927

Short period sdB binaries with cool companions are crucial to understand pre-CV evolution, because they will evolve into cataclysmic variables, when the sdB will have left the extended horizontal branch. Recently we discovered the sixth such system, HS 2333+3927, consisting of an sdB star and an M dwarf (period: 0.172 d) with a very strong re°ection e®ect, but no eclipses. The re°ection is stronger than in any of the other similar systems which renders a quantitative spectral analysis very di±cult because the Balmer line pro¯les may be disturbed by the re°ected light. A spectroscopic analysis results in Te® = 36 500 K, log g = 5.70, and log(nHe=nH) = ¡2:15. Mass-radius relations were derived from the results of the analysis of light and radial-velocity curves. Comparison with the mass-radius relation derived from the surface gravity of the sdB star favours a rather low mass of 0.38 M¯ for the primary. The mass of the companion is 0.29 M¯. HS 2333+3927 is the only known sdB+dM system with a period above the CV period gap.