The BF Aurigae system - A close binary at the onset of mass transfer

Because the question of the mass ratio of the early-type system BF Aur has not yet fully been clarified, we present and analyse new UBV photometry and nearly contemporaneous radial velocity observations. From a simultaneous l east squares analysis of the photometric light curves and the new radial ve locity curves we derive a mass ratio of q = 1.048 +/- 0.02. With even much more accurate photometric and spectroscopic data the mass ratio remains wea kly defined. The resulting stellar parameters are consistent with line rati os derived from old and our new spectra and available Stromgren indices. We confirm earlier conclusions that BF Aur is detached and that the more mass ive component is almost filling its Roche lobe. New data indicating a perio d change support the interpretation that mass exchange has already started or is about to start.