  We report results of an extensive world-wide observing campaign devoted to
the recently discovered dwarf nova SDSS J162520.29+120308.7 (SDSS J1625). The
data were obtained during the July 2010 eruption of the star and in August and
September 2010 when the object was in quiescence. During the July 2010
superoutburst SDSS J1625 clearly displayed superhumps with a mean period of
$P_{\rm sh}=0.095942(17)$ days ($138.16 \pm 0.02$ min) and a maximum amplitude
reaching almost 0.4 mag. The superhump period was not stable, decreasing very
rapidly at a rate of $\dot P = -1.63(14)\cdot 10^{-3}$ at the beginning of the
superoutburst and increasing at a rate of $\dot P = 2.81(20)\cdot 10^{-4}$ in
the middle phase. At the end of the superoutburst it stabilized around the
value of $P_{\rm sh}=0.09531(5)$ day. During the first twelve hours of the
superoutburst a low-amplitude double wave modulation was observed whose
properties are almost identical to early superhumps observed in WZ Sge stars.
The period of early superhumps, the period of modulations observed temporarily
in quiescence and the period derived from radial velocity variations are the
same within measurement errors, allowing us to estimate the most probable
orbital period of the binary to be $P_{\rm orb}=0.09111(15)$ days ($131.20 \pm
0.22$ min). This value clearly indicates that SDSS J1625 is another dwarf nova
in the period gap. Knowledge of the orbital and superhump periods allows us to
estimate the mass ratio of the system to be $q\approx 0.25$. This high value
poses serious problems both for the thermal and tidal instability (TTI) model
describing the behaviour of dwarf novae and for some models explaining the
origin of early superhumps.
