Spectroscopic and photometric periods of six ultracompact accreting Binaries

Ultracompact accreting binary systems each consist of a stellar remnant accreting heliumenriched material from a compact donor star. Such binaries include two related sub-classes, AMCVn-type binaries and helium cataclysmic variables, in both of which the central star is a white dwarf. We present a spectroscopic and photometric study of six accreting binaries with orbital periods in the range of 40-70 min, including phase-resolved VLT spectroscopy and high-speed ULTRACAM photometry. Four of these are AMCVn systems and two are helium cataclysmic variables. For four of these binarieswe are able to identify orbital periods (ofwhich three are spectroscopic). SDSS J1505+0659 has an orbital period of 67.8min, significantly longer than previously believed, and longer than any other known AMCVn binary. We identify a Wide-field Infrared Survey Explorer (WISE) infrared excess in SDSS J1505+0659 that we believe to be the first direct detection of an AMCVn donor star in a non-direct impacting binary. The mass ratio of SDSS J1505+0659 is consistent with a white dwarf donor. CRTS J1028-0819 has an orbital period of 52.1min, the shortest period of any helium cataclysmic variable. MOA2010-BLG-087 is co-aligned with a K-class star that dominates its spectrum. ASASSN-14ei and ASASSN-14mv both show a remarkable number of echo outbursts following superoutbursts (13 and 10 echo outbursts respectively). ASASSN-14ei shows an increased outburst rate over the years following its superoutburst, perhaps resulting from an increased accretion rate.