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We investigate the properties of 367 ultra-short period binary candidatesselected from 31,000 sources recently identified from Catalina Surveys data.Based on light curve morphology, along with WISE, SDSS and GALEX multi-colourphotometry, we identify two distinct groups of binaries with periods below the0.22 day contact binary minimum. In contrast to most recent work, wespectroscopically confirm the existence of M-dwarf+M-dwarf contact binarysystems. By measuring the radial velocity variations for five of theshortest-period systems, we find examples of rare cool-white dwarf+M-dwarfbinaries. Only a few such systems are currently known. Unlike warmer whitedwarf systems, their UV flux and their optical colours and spectra aredominated by the M-dwarf companion. We contrast our discoveries with previousphotometrically-selected ultra-short period contact binary candidates, andhighlight the ongoing need for confirmation using spectra and associated radialvelocity measurements. Overall, our analysis increases the number ofultra-short period contact binary candidates by more than an order ofmagnitude.

Ultra-short Period Binaries from the Catalina Surveys