  We found that SDSS J094002.56+274942.0 underwent a superoutburst in 2019
February based on our observations and Zwicky Transient Facility (ZTF) data.
This object showed shallow eclipses during this superoutburst and we
established the orbital period to be 0.1635015(1) d in combination with the ZTF
and Asteroid Terrestrial-impact Last Alert System (ATLAS) data in quiescence.
Superhumps apparently started to develop soon after the object reached the
plateau phase and fully grown superhumps were recorded within the initial 6 d
of the plateau phase. Using the superhump and orbital periods, we obtained a
mass ratio (q) of 0.39(3) and obtained an inclination of 70.5(5) deg by eclipse
modeling. These values reproduced the quiescent ellipsoidal variations very
well. Using the Gaia parallax and 2MASS observations, we confirmed that the
secondary is indistinguishable from an unevolved main-sequence star. The
resultant mass ratio and orbital period were the highest among SU UMa stars,
and this provided a proof that the 3:1 resonance can develop in less than 6 d
even in q=0.39(3). The superoutburst faded relatively rapidly and was followed
by a rebrightening, suggesting that the tidal effect in a large-q system was
insufficient to maintain a long superoutburst and the remnant matter caused a
rebrightening. The presence of such a system among dwarf novae is against the
conventional idea that outbursts in dwarf novae are not long enough to develop
superhumps, in contrast to novalike variables, under a weak tidal effect. The
present observation also supports that the 3:1 resonance is the cause of a long
outburst, and not its consequence, even under extreme q. The rapid growth of
the 3:1 resonance in a high-q system challenges the generally accepted results
of hydrodynamic simulations.
