We investigate the properties of 367 ultra-short period binary candidates selected from 31,000 sources recently identified from Catalina Surveys data. Based on light curve morphology, along with WISE, SDSS and GALEX multi-colour photometry, we identify two distinct groups of binaries with periods below the 0.22 day contact binary minimum. In contrast to most recent work, we spectroscopically confirm the existence of M-dwarf+M-dwarf contact binary systems. By measuring the radial velocity variations for five of the shortest-period systems, we find examples of rare cool-white dwarf+M-dwarf binaries. Only a few such systems are currently known. Unlike warmer white dwarf systems, their UV flux and their optical colours and spectra are dominated by the M-dwarf companion. We contrast our discoveries with previous photometrically-selected ultra-short period contact binary candidates, and highlight the ongoing need for confirmation using spectra and associated radial velocity measurements. Overall, our analysis increases the number of ultra-short period contact binary candidates by more than an order of magnitude.
@article{arxiv.1406.4504,
title = {Ultra-short Period Binaries from the Catalina Surveys},
author = {A. J. Drake and S. G. Djorgovski and D. Garcia-Alvarez and M. J. Graham and M. Catelan and A. A. Mahabal and C. Donalek and J. L. Prieto and G. Torrealba and S. Abraham and R. Williams and S. Larson and E. Christensen},
journal= {arXiv preprint arXiv:1406.4504},
year = {2015}
}