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 reflection effect , but no eclipses .
The reflection is stronger than in any of the other similar systems which renders a quantitative spectral analysis very difficult because the Balmer line profiles may be disturbed by the reflected light .
A spectroscopic analysis results in { T _ { eff } } = 36 500 K , \log { g } = 5.70 , and \log ( n _ { He } / n _ { H } ) = -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 _ { \odot } for the primary .
The mass of the companion is 0.29 M _ { \odot } .
HS 2333+3927 is the only known sdB+dM system with a period above the CV period gap .