The shortest-period M-dwarf eclipsing system: BW3 V38

The photometric data for a short-period (0.1984 day) eclipsing binary V38 discovered by the OGLE microlensing team in Baade's Window field BW3 have been analyzed. The light-curve synthesis solution of the I-filter light curve and dereddened color (V-I-C)(0) = 2.3 suggest a pair of strongly distorted M dwarfs, with parameters between those of YY Gem and CM Dra, revolving on the lightest known orbit among binaries consisting of main-sequence stars. The primary, more massive and hotter, component may be filling its Roche lobe. The very small amount of angular momentum in the orbital motion makes the system particularly important for studies of angular-momentum loss at the faint end of the main sequence. Spectroscopic observations of the orbital radial-velocity variations as well as of activity indicators are urgently needed for a better understanding of the angular momentum, internal structure, and evolutionary stale of the system.