Neustroev, V.V, Marsh, T.R., Zharikov, S.V., Knigge, C., Kuulkers, E., Osborne, J.P., Page, K.L., Steeghs, D., Suleimanov, V.F., Tovmassian, G., Breedt, E., Frebel, A., Garcia-Diaz, Ma. T., Hambsch, F.-J., Jacobson, H., Parsons, S.G., Ryu, T., Sabin, L., Sjoberg, G., Miroshnichenko, A.S., Reichart, D.E., Haislip, J.B., Ivarsen, K.M., LaCluyze, A.P. and Moore, J.P. (2017) The remarkable outburst of the highly evolved post-period-minimum dwarf nova SSS J122221.7−311525⋆. Monthly Notices of the Royal Astronomical Society, 467 (1), 597-618. (doi:10.1093/mnras/stx084).
Abstract
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 ≃ 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 ≲0.8 per cent, indicating a very low mass ratio q ≲ 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.
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