* Astronomy

Title: SDSS J210014.12+004446.0 - dwarf nova with negative and positive superhumps
Authors: A. Olech, A. Rutkowski, A. Schwarzenberg-Czerny

We report the results of 67h of CCD photometry of the recently discovered dwarf nova SDSS J210014.12+004446.0. 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. These humps were already observed on one night by Tramposch et al. (2005), 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. Additionally, we have promoted SDSS J2100 to the 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. 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(3)% and -2.660(8)%, yielding the mass ratio q=0.24.

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