We present outburst spectroscopy of GX 339–4 which may reveal the motion of its elusive companion star .
N iii lines exhibit sharp emission components moving over \sim 300 km s ^ { -1 } in a single night .
The most plausible interpretation of these components is that they are formed by irradiation of the companion star and the velocities indicate its orbital motion .
We also detect motion of the wings of the He ii 4686 Å line and changes in its morphology .
No previously proposed period is consistent with periodic behavior of all of these measures .
However , consistent and sensible solutions are obtained for periods around 1.7 days .
For the best period , 1.7557 days , we estimate a mass function of 5.8 \pm 0.5 M _ { \odot } .
Even allowing for aliases , the 95 % confidence lower-limit on the mass function is 2.0 M _ { \odot } .
GX 339–4 can therefore be added to the list of dynamical black hole candidates .
This is supported by the small motion in the wings of the He ii line ; if the compact object velocity is not larger than the observed motion then the mass ratio is q \lesssim 0.08 , similar to other systems harboring black holes .
Finally , we note that the sharp components are not always present , but do seem to occur within a repeating phase range .
This appears to migrate between our epochs of observation , and may indicate shielding of the companion star by a variable accretion geometry such as a warp .