Aims: Large radial velocity variations in the LAMOST spectra of giant stars have been used to infer the presence of unseen companions. Some of them have been proposed as possible black hole candidates. We test this selection by investigating the classification of the one candidate that has a known X-ray counterpart (UCAC4 721-037069).
Methods: We obtained time-resolved spectra from the Liverpool Telescope and a 5 ks observation from the Chandra observatory to fully constrain the orbital parameters and the X-ray emission of this system.
Results: We find the source to be an eclipsing stellar binary that can be classified as an RS CVn. The giant star fills its Roche lobe, and the binary mass ratio is greater than one. The system may be an example of stable mass transfer from an intermediate-mass star with a convective envelope.
Conclusions: Using only radial velocity to identify black hole candidates can lead to many false positives. The presence of an optical orbital modulation, such as what has been observed for all LAMOST candidates, will in most cases indicate that the system is a stellar binary.

Radial velocities are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/645/A72