Curious Variables Experiment: SDSS J210014.12+004446.0 - dwarf nova with negative and positive superhumps

We report the results of 67 h of CCD photometry of the recently discovered dwarf nova SDSS J210014.12+004446.0 (SDSS J2100). The data were obtained on 24 nights spanning a month. During this time, we observed four ordinary outbursts lasting about 2-3 days and reaching an amplitude of ~1.7 mag. On all nights, our light curve revealed persistent modulation with the stable period of 0.081088(3) days (116.767 +/- 0.004 min) and large amplitude of 0.5-0.6 mag in quiescence reduced to 0.1-0.2 mag during outbursts.

These humps were already observed on one night by Tramposch et al., who additionally observed superhumps during a superoutburst. Remarkably, from scant evidence at their disposal they were able to discern them as negative and positive (common) superhumps, respectively. Our period in quiescence clearly different from their superhump period confirmed this. Our discovery of an additional modulation, attributed by us to the orbital wave, completes the overall picture. Lack of superhumps in our data indicates that all eruptions we observed were ordinary outbursts. The earlier observation of the superhumps combined with the presence of the ordinary outbursts in our data enables classification of SDSS J2100 as an active SU UMa dwarf nova with two types of outbursts.

Additionally, we have promoted SDSS J2100 to the select group of cataclysmic variables exhibiting three periodic modulations of light from their accretion discs. We updated available information on positive and negative superhumps and thus provided enhanced evidence that their properties are strongly correlated mutually as well as with the orbital period. By recourse to these relations, we were able to remove an alias ambiguity and to identify the orbital period of SDSS J2100 of 0.083304(6) days (119.958 +/- 0.009 min). SDSS J21000 is only third SU UMa dwarf nova showing both positive and negative superhumps. Their respective period excess and deficit equal to 4.99 +/- 0.03 and -2.660 +/- 0.008 per cent, yielding the mass ratio q ~ 0.24.