We have used innovative features of the Taurus Tunable Filter instrument on the 3.9-m Anglo-Australian Telescope to obtain nearly-continuous , high-throughput , linear photometry of V2116 Oph in a 7 Å bandpass at the center of the O I \lambda 8446 emission line .
This instrumental technique shows promise for applications requiring precise , rapid , narrowband photometry of faint objects .

The spectrum of V2116 Oph , the counterpart of GX 1+4 ( =X1728–247 ) , is exotic , even among the unusual spectra of other optical counterparts of compact Galactic X-ray sources .
The second strongest emission line is an unusual one , namely extremely prominent O I \lambda 8446 , which is likely to result from pumping by an intense Ly \beta radiation field .
As the X-radiation from GX 1+4 is steadily pulsed , with typical pulsed fractions of 0.4 , the O I \lambda 8446 emission in V2116 Oph may also be strongly modulated with the current 127 s period of the X-ray source .
If so , this may well allow us to obtain high signal-to-noise radial velocity measurements and thus to determine the system parameters .
However , no such pulsations are detected , and we set an upper limit of \sim 1 % ( full-amplitude ) on periodic \lambda 8446 oscillations at the X-ray frequency .
This value is comparable to the amplitude of continuum oscillations observed on some nights by other workers .
Thus we rule out an enhancement of the pulsation amplitude in O I emission , at least at the time of our observations .