We present time-resolved spectroscopy and photometry of CSS 120422:111127+571239 (= SBS 1108+574), a recently discovered SU UMatype dwarf nova whose 55-minute orbital period is well below the CV period minimum of ∼78 minutes. In contrast with most other known CVs, its spectrum features He I emission of comparable strength to the Balmer lines, implying a hydrogen abundance less than 0.1 of long period CVs-but still at least 10 times higher than than in AM CVn stars. Together, the short orbital period and remarkable helium-to-hydrogen ratio suggest that mass transfer in CSS 120422 began near the end of the donor star's main-sequence lifetime, meaning that the system is probably an AM CVn progenitor as described by Podsiadlowski, Han, & Rappaport (2003). Moreover, a Doppler tomogram of the Hα line reveals two distinct regions of enhanced emission. While one is the result of the stream-disk impact, the other is probably attributable to spiral disk structure generated when material in the outer disk achieves a 2:1 orbital resonance with respect to the donor.
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