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, is 65.233 ± 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 AM CVn-type stars. Comparing their properties with cataclysmic variables (CVs), AM CVn systems and hydrogen binaries below the CV period minimum, we argue that CSS100603:112253−111037 is the first compelling example of an AM CVn system forming via the evolved CV channel.
With the addition of this system, evolved CVs now account for 7 per cent of all known semidetached white dwarf binaries with Porb < 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 AM CVn stars, but there are currently no known systems in the period range of 67–76 min.