We present time-resolved spectroscopy and photometry of the dwarf nova SBSS 1108+574, obtained during the 2012 outburst. Its quiescent spectrum is unusually rich in helium, showing broad, double-peaked emission lines from the accretion disc. We measu
re a line flux ratio HeI 5875/Halpha = 0.81 +/- 0.04, a much higher ratio than typically observed in cataclysmic variables (CVs). The outburst spectrum shows hydrogen and helium in absorption, with weak emission of Halpha and HeI 6678, as well as strong HeII emission. From our photometry, we find the superhump period to be 56.34 +/- 0.18 minutes, in agreement with the previously published result. The spectroscopic period, derived from the radial velocities of the emission lines, is found to be 55.3 +/- 0.8 minutes, consistent with a previously identified photometric orbital period, and significantly below the normal CV period minimum. This indicates that the donor in SBSS 1108+574 is highly evolved. The superhump excess derived from our photometry implies a mass ratio of q = 0.086 +/- 0.014. Our spectroscopy reveals a grazing eclipse of the large outbursting disc. As the disc is significantly larger during outburst, it is unlikely that an eclipse will be detectable in quiescence. The relatively high accretion rate implied by the detection of outbursts, together with the large mass ratio, suggests that SBSS 1108+574 is still evolving towards its period minimum.
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 whe
n 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 $qapprox 0.25$. 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.
