The shortest period M dwarf eclipsing system BW3 V38 II. Determination of absolute elements

The spectroscopic data for the short-period (0d.1984) eclipsing binary V38, discovered by the OGLE micro-lensing team in Baade's Window field BW3, are analyzed. Radial velocity curves are derived from mid-resolution spectra obtained with EMMI-NTT at ESO - La Silla, and a simultaneous solution of the existing light curve by OGLE and of the new radial velocity curves is obtained. The system is formed by almost twin M3e dwarf components that are very close, but not yet in contact. The spectra of both dwarfs show signatures of the presence of strong chromospheres. Spectroscopy definitely confirms, therefore, what was suggested on the basis of photometry: BW3 V38 is indeed a unique system, as no other similar binary with M components and in such a tight orbit is known. Within the limits posed by the relatively large errors, due to the combined effect of system faintness and of the constraints on exposure time, the derived physical parameters seem to agree with the relations obtained from the other few known eclipsing binaries with late type components (which indicate a discrepancy between the available evolutionary models and the data at ∼10% level). A possible explanation is the presence of strong magnetic fields and fast rotation (that applies to the BW3 V38 case as well). A simple computation of the system secular evolution by angular momentum loss and spin-orbit synchronization shows that the evolution of a system with M dwarf components is rather slow, and indicates as well a possible reason why systems similar to BW3 V38 are so rare.