SIMBAD references

The lower tail of the period distribution of main-sequence binary stars is poorly constrained due to the small number of such systems known. This paper deals with candidate binary stars with periods shorter than 0.2 d and colour indices consistent with them being M dwarfs. Three sources of candidates are considered - a catalogue of TESS eclipsing binaries, the ATLAS catalogue of variable stars, and binary stars from the 'International Variable Star Index' (VSX) website. The TESS eclipsing binary catalogue contains five new candidates satisfying the period and colour criteria. New photometry confirms one as a bona fide main-sequence system, and another as a white dwarf/red dwarf pair. New multicolour light curves of 10 ATLAS stars are presented: 7 of these are main sequence binaries with confirmed ultrashort periods (P < 0.2 d). There are 328 red VSX binary candidates with short periods. Literature searches for 77 of these with P < 0.17 d were performed: only 6 are previously studied main sequence systems, and another 14 are known red dwarf/white dwarf systems.