A new sdO+dM binary with extreme eclipses and reflection effect

We report the discovery of a new totally eclipsing binary (R.A. = 06(h)40(m)29(s)11; decl. = + 38 degrees 56'52 " 2; J = 2000.0; R-max = 17.2 mag) with an sdO primary and a strongly irradiated red dwarf companion. It has an orbital period of P-orb = 0.187284394(11) day and an optical eclipse depth in excess of 5 mag. We obtained 2 low-resolution classification spectra with GTC/OSIRIS and 10 medium-resolution spectra with WHT/ISIS to constrain the properties of the binary members. The spectra are dominated by H Balmer and He II absorption lines from the sdO star, and phase-dependent emission lines from the irradiated companion. A combined spectroscopic and light curve analysis implies a hot subdwarf temperature of T-eff(spec) = 55,000 +/- 3000 K, surface gravity of log g (phot) = 6.2 +/- 0.04 (cgs), and a He abundance of log(nHenH) = -2.24 +/- 0.40. The hot sdO star irradiates the red dwarf companion, heating its substellar point to about 22,500 K. Surface parameters for the companion are difficult to constrain from the currently available data: the most remarkable features are the strong H Balmer and C II-III lines in emission. Radial velocity estimates are consistent with the sdO+dM classification. The photometric data do not show any indication of sdO pulsations with amplitudes greater than 7 mmag, and H alpha-filter images do not provide evidence for the presence of a planetary nebula associated with the sdO star.