Ultra-short period contact binaries: restricting the parameters of the primary using Gaia parallax

Possible M-type contact binaries were investigated by selecting W UMa-type variables with orbital periods below the 0.22-day cutoff. Gaia parallaxes were combined with Gaia and 2MASS photometry to obtain G and J-band absolute magnitudes of 674 red variable stars catalogued as contact binaries in the VSX database. The absolute magnitude of main sequence cool dwarfs varies strongly with spectral type, which was used to create a selection of 218 systems with primaries potentially of spectral types M0-M3. Lightcurves of the 46 systems with lowest near-infrared luminosities were inspected individually to confirm or reject the W UMa classification where possible. The extinction limits and amplitudes were combined with the calculated absolute magnitudes in order to set upper limits for the masses of the primary components. This is achieved via the consideration that the luminosity of the primary is no less than that of a main sequence star of the same mass. For 26 possible contact binaries the mass of the primary was limited to less than 0.5 solar masses with some systems having upper limits for the primary as low as 0.35 solar masses. These rare systems are intriguing targets for more detailed observations since they are on the low-mass end of the contact binary distribution and their formation and evolution are still unclear.