  We report on a superoutburst of the AM CVn-type object SDSS
J090221.35+381941.9 [J0902; orbital period 0.03355(6) d] in 2014 March-April.
The entire outburst consisted of a precursor outburst and the main
superoutburst, followed by a short rebrightening. During the rising branch of
the main superoutburst, we detected growing superhumps (stage A superhumps)
with a period of 0.03409(1) d. During the plateau phase of the superoutburst,
superhumps with a shorter period (stage B superhumps) were observed. Using the
orbital period and the period of the stage A superhumps, we were able to
measure the dynamical precession rate of the accretion disk at the 3:1
resonance, and obtained a mass ratio (q) of 0.041(7). This is the first
successful measurement of the mass ratio in an AM CVn-type object using the
recently developed stage A superhump method. The value is generally in good
agreement with the theoretical evolutionary model. The orbital period of J0902
is the longest among the outbursting AM CVn-type objects, and the borderline
between the outbursting systems and systems with stable cool disks appears to
be longer than had been supposed.
