CSS100603: 112253-111037: A helium-rich dwarf nova with a 65 min orbital period

We present time-resolved optical spectroscopy of the dwarf nova CSS100603: 112253-111037. Its optical spectrum is rich in helium, with broad, double-peaked emission lines produced in an accretion disc.We measure a line flux ratio He I λ5876/Hα = 1.49 ± 0.04, a much higher ratio than is typically observed in dwarf novae. The orbital period, as derived from the radial velocity of the line wings, is65.233 ± 0.015 min. In combination with the previously measured superhump period, this implies an extreme mass ratio of M₂/M₁ = 0.017 ± 0.004. The Hα and He I λ 6678 emission lines additionally have a narrow central spike, as is often seen in the spectra of AMCVn-type stars. Comparing their properties with cataclysmic variables (CVs), AMCVn systems and hydrogen binaries below the CV period minimum, weargue that CSS100603:112253-111037 is the first compelling example of an AMCVn system forming via theevolved CV channel. With the addition of this system, evolved CVs now account for 7 per cent of all known semidetached white dwarf binaries with P_orb < 76 min. Two recently discovered binaries may further increase this figure. Although the selection bias of this sample is not yet well defined, these systems support the evolved CV model as a possible formation channel for ultracompact accreting binaries. The orbital periods of the three ultracompact hydrogen accreting binaries overlap with those of the long-period AMCVn stars, but there are currently no known systems in the period range of 67-76 min.