We report extensive 3-yr multiwavelength observations of the WZ Sge-type dwarf nova SSS J122221.7 - 311525 during its unusual double superoutburst , the following decline and in quiescence .
The second segment of the superoutburst had a long duration of 33 d and a very gentle decline with a rate of 0.02 mag d ^ { -1 } , and it displayed an extended post-outburst decline lasting at least 500 d. Simultaneously with the start of the rapid fading from the superoutburst plateau , the system showed the appearance of a strong near-infrared excess resulting in very red colours , which reached extreme values ( B - I \simeq 1.4 ) about 20 d later .
The colours then became bluer again , but it took at least 250 d to acquire a stable level .
Superhumps were clearly visible in the light curve from our very first time-resolved observations until at least 420 d after the rapid fading from the superoutburst .
The spectroscopic and photometric data revealed an orbital period of 109.80 min and a fractional superhump period excess \lesssim 0.8 per cent , indicating a very low mass ratio q \lesssim 0.045 .
With such a small mass ratio the donor mass should be below the hydrogen-burning minimum mass limit .
The observed infrared flux in quiescence is indeed much lower than is expected from a cataclysmic variable with a near-main-sequence donor star .
This strongly suggests a brown-dwarf-like nature for the donor and that SSS J122221.7 - 311525 has already evolved away from the period minimum towards longer periods , with the donor now extremely dim .