The discovery of RE 1307 + 535: the shortest period AM Her system

We report on a ROSAT Wide Field Camera EUV survey observation of RE 1307 + 535 which, together with optical spectroscopic and photometric observations, shows that this object is an AM Her system. These observations reveal that RE 1307 + 535 has an orbital period of 79.69 min, the shortest orbital period known for such a system. RE 1307 + 535 was observed by us in both high (V = 17–18) and low (V = 20–21) states, with evidence of cyclotron emission being seen in both cases. We use the high-state spectroscopy to show that the optical emission comes from a region with a magnetic field, B ≈ 30–40 MG. We derive EUV temperature and luminosity limits. IRCAM infrared photometry is used to put a lower limit on the distance to RE 1307 + 535 of d > 705 pc. RE 1307 + 535 is thus one of the most distant AM Her systems known, and is the first that is at a measured distance of z > 630 pc above the Galactic plane.