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We report extensive 3-yr multiwavelength observations of the WZ Sge-typedwarf nova SSS J122221.7-311525 during its unusual double superoutburst, thefollowing decline and in quiescence. The second segment of the superoutbursthad a long duration of 33 d and a very gentle decline with a rate of 0.02mag/d, and it displayed an extended post-outburst decline lasting at least 500d. Simultaneously with the start of the rapid fading from the superoutburstplateau, the system showed the appearance of a strong near-infrared excessresulting in very red colours, which reached extreme values (B-I~1.4) about 20d later. The colours then became bluer again, but it took at least 250 d toacquire a stable level. Superhumps were clearly visible in the light curve fromour very first time-resolved observations until at least 420 d after the rapidfading from the superoutburst. The spectroscopic and photometric data revealedan orbital period of 109.80 min and a fractional superhump period excess <0.8per cent, indicating a very low mass ratio q<0.045. With such a small massratio the donor mass should be below the hydrogen-burning minimum mass limit.The observed infrared flux in quiescence is indeed much lower than is expectedfrom a cataclysmic variable with a near-main-sequence donor star. This stronglysuggests a brown-dwarf-like nature for the donor and that SSS J122221.7-311525has already evolved away from the period minimum towards longer periods, withthe donor now extremely dim.

The remarkable outburst of the highly evolved post-period-minimum dwarf nova SSS J122221.7-311525