1H 0709-360 : a cataclysmic variable with an orbital period inside the period gap

The discovery is reported of the second cataclysmic variable with a binary period lying well inside the nominal 2-3 hr period gap. The object, 1H 0709-360, is an X-ray source which has been identified on the basis of positional data from the instruments of the HEAO 1 survey. The X-ray source was also detected by the Uhuru and Ariel 5 surveys. Multicolor photometry has revealed eclipsing behavior at an orbital period of 2.444 hr. Radial velocity measurements show modulations at the same period, with an offset between the spectroscopic and photometric conjunctions. Both the eclipse profile and a clear rotational disturbance in the radial velocities of emission lines during eclipse provide unambiguous evidence for the existence of an accretion disk. Thus the object cannot be an AM Herculis variable, a conclusion supported by the absence of circular polarization. Instead, the evidence suggests that the object could be a DQ Herculis magnetic variable that is close to synchronism. The evolutionary implications of this rare object are discussed. 41 refs.