MN Draconis: a peculiar, active dwarf nova in the period gap

Context. We present results of an extensive world-wide observing campaign of MN Draconis.
Aims: MN Draconis is a poorly known active dwarf nova in the period gap and is one of only two known cases of period-gap SU UMa objects showing negative superhumps. The photometric behaviour of MN Draconis poses a challenge for existing models of the superhump and superoutburst mechanisms. Therefore, a thorough investigation of peculiar systems like MN Draconis is crucial for our understanding of the evolution of close binary stars.
Methods: To measure the fundamental parameters of the system, we collected photometric data in October 2009, June-September 2013, and June-December 2015. We analysed the light curves, O-C diagrams, and power spectra.
Results: During our three observational seasons we detected four superoutburts and several normal outbursts. Based on the two consecutive superoutbursts detected in 2015, the supercycle length was derived as P_sc = 74 ± 0.5 days, and it has been increasing with a rate of ^˙P = 3.3 × 10^-3 during the past twelve years. Based on the positive and negative superhumps, we calculated the period excess ɛ = 5.6% ± 0.1%, the period deficit ɛ_- = 2.5% ± 0.6%, and as a result, the orbital period P_orb = 0.0994(1) days (143.126 ± 0.144 min). We updated the basic light curve parameters of MN Draconis.
Conclusions: MN Draconis is the first discovered SU UMa system in the period gap with an increasing supercycle length.

Photometric data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/603/A72