1999A&A...352L..95G

We have discovered periodic changes in velocity and equivalent width of a number of different spectral features of RW Aur A, a T Tauri star with a jet, broad emission lines and substantial excess continuous emission. In particular, the photospheric lines vary in radial velocity with an amplitude of 5.7km/s over a period of 2.77 days, which appears to have been stable over at least 18 years. The present Letter focusses solely on one straightforward interpretation, namely that RW Aur A is a single-lined spectroscopic binary. However, other spectral features vary in or out of phase with the same or the double period. Interpretations, other than a binary model, should be considered but must await the more extended report on our large observational material. In the binary case, the small amplitude leads to a mass function of only 5.10^–5M_☉, by far the smallest observed for young binaries. The jet velocity and extent restrict the range of inclinations (if the orbital axis is aligned with the jet), implying a close binary with a secondary at the brown dwarf limit. Further restrictions follow if the primary rotates synchronously. Narrow emission components of HeI and HeII vary in anti-phase with the photospheric absorption lines. We show that it is unlikely that these lines reflect the motion of the secondary.