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) over dot = 3.3 x 10(-3) during the past twelve years. Based on the positive and negative superhumps, we calculated the period excess epsilon = 5.6% +/- 0.1%, the period deficit epsilon(-) = 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.