OGLE-BLG504.12.201843: A possible extreme dwarf nova

We present the analysis of photometry and spectroscopy of the candidate cataclysmic variable star OGLE-BLG504.12.201843. The system undergoes 300 days long outbursts recurring every 1000 days, and shows a slow brightnening accompanied by small flares during quiesence. The orbital variability of the system in outburst shows clear signs of an accretion disc, from which the outburst likely originates. The spectra show Balmer absorption lines both in quiescence and outburst, which can be associated with a bright companion star or a cold accretion disc. During outbursts, emission lines appear; however, they lack typical double-peaked profiles. We suggest that this object is an extreme dwarf nova. Due to its peculiarities, OGLE-BLG504.12.201843 is an excellent object for further studies of dwarf novae.