Post-common envelope binaries from SDSS - III. Seven new orbital periods

We present follow-up spectroscopy and photometry of 11 post-common envelope binary (PCEB) candidates identified from multiple Sloan Digital Sky Survey (SDSS) spectroscopy in an earlier paper. Radial velocity measurements using the Na I lambda lambda 8183.27, 8194.81 absorption doublet were performed for nine of these systems and provided measurements of six orbital periods in the range P(orb) = 2.7 - 17.4 h. Three PCEB candidates did not show significant radial velocity variations in the follow-up data, and we discuss the implications for the use of SDSS spectroscopy alone to identify PCEBs. Differential photometry confirmed one of our spectroscopic orbital periods and provided one additional P(orb) measurement. Binary parameters are estimated for the seven objects for which we have measured the orbital period and the radial velocity amplitude of the low-mass companion star, K(sec). So far, we have published nine SDSS PCEBs orbital periods, all of them P(orb) < 1 d. We perform Monte Carlo simulations and show that 3 sigma SDSS radial velocity variations should still be detectable for systems in the orbital period range of P(orb) similar to 1 - 10 d. Consequently, our results suggest that the number of PCEBs decreases considerably for P(orb) > 1 d, and that during the CE phase the orbital energy of the binary star is may be less efficiently used to expel the envelope than frequently assumed.