Orbital periods of cataclysmic variables identified by the SDSS - II. Measurements for six objects, including two eclipsing systems

Abstract

Continuing our work from Paper I (Southworth et al., 2006) we present medium-resolution spectroscopy and broad-band photometry of seven cataclysmic variables (CVs) discovered by the SDSS. For six of these objects we derive accurate orbital periods, all which are measured for the first time. For SDSS J013132.39+090122.2, which contains a non-radially pulsating white dwarf, we find an orbital period of 81.54 +/- 0.13 min and a low radial velocity variation amplitude indicative of an extreme mass ratio. For SDSS J205914.87+061220.4, we find a period of 107.52 +/- 0.14 min. This object is a dwarf nova and was fading from its first recorded outburst throughout our observations. INT photometry of SDSS J155531.99-001055.0 shows that this system undergoes total eclipses which are 1.5 mag deep and occur on a period of 113.54 +/- 0.03 min. A NOT light curve of SDSS J075443.01+500729.2 shows that this system is also eclipsing, on a period of 205.965 +/- 0.014 min, but here the eclipses are V-shaped and only 0.5 mag deep. Its low emission-line strengths, orbital period and V-shaped eclipse unambiguously mark it as a novalike object. WHT photometry of SDSS J005050.88+000912.6 and SDSS J210449.94+010545.8 yields periods of 80.3 +/- 2.2 and 103.62 +/- 0.12 min, respectively. Photometry of the seventh and final system, SDSS J165658.12+212139.3, shows only flickering. Our results strengthen the conclusion that the faint magnitude limit of the SDSS spectroscopic database implies that the sample of CVs contained in it has quite different characteristics to previously studied samples of these objects. Five of the six orbital periods measured here are shorter than the observed 2-3 hr CV period gap. Two systems have periods very close to the minimum orbital period for hydrogen-rich CVs.