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We present the analysis of existing optical photometry and new opticalspectroscopy of the candidate cataclysmic variable star OGLE-BLG504.12.201843.As was shown previously, this object has an orbital period of 0.523419 days andexhibits year-long outbursts with a mean period of 973 days. Using digitizedphotographic archives, we show that the earliest recorded outburst occurred in1910. We propose that this object is a U Gem-type dwarf nova with extremeproperties. The orbital variability of the system in outburst shows clear signsof an accretion disc, from which the outburst likely originates. Duringquiescence, the object slowly brightens by up to $0.75$ mag in the $I$ bandover 600 days before the outburst and exhibits small flares with amplitude$\lesssim 0.2$ mag in the $I$ band. We interpret the gradual brightening as anincrease in the luminosity and temperature of the accretion disc, which istheoretically predicted but only rarely seen in DNe. The origin of small flaresremains unexplained. The spectra shows Balmer absorption lines both inquiescence and outburst, which can be associated with a bright secondary staror a cold accretion disc. During outbursts, emission lines with FWHM of about450 km s$^{-1}$ appear, but they lack typical double-peaked profiles. Wesuggest that either these lines originate in the disc winds or the orbitalinclination is low, the latter being consistent with constrains obtained fromthe orbital variability of the system. Due to its extreme properties andpeculiarities, OGLE-BLG504.12.201843 is an excellent object for furtherfollow-up studies.

OGLE-BLG504.12.201843: A possible extreme dwarf nova