Discovery of a Dwarf Nova Breaking the Standard Sequence of Compact Binary Evolution

We revealed that the dwarf nova 1RXS J232953.9+062814 is an SU UMa-type system with a superhump period of 66.774+/-0.010 min. The short period strongly indicates that the orbital period of this object is below the period minimum of cataclysmic variables. The superhump period is 4.04+/-0.02 % longer than the photometric period during quiescence (64.184+/-0.003 min), which is probably associated with the orbital period. Although the standard evolutionary scenario of cataclysmic variables predicts lower mass-transfer rates in systems with shorter orbital periods, we revealed firm evidence of a relatively high mass-transfer rate from its large proper motion and bright apparent magnitude. Its proximity indicates that we have overlooked a number of objects in this new class. With the analogous system of V485 Cen, these objects establish the first subpopulation in hydrogen-rich cataclysmic variables below the period minimum.