We present evidence for approximately 400-d variations in the radial velocity of HD 154791 ( V934 Her ) , the suggested optical counterpart of 4U 1700+24 .
The variations are correlated with the previously reported \approx 400 d variations in the X-ray flux of 4U 1700+24 , which supports the association of these two objects , as well as the identification of this system as the second known X-ray binary in which a neutron star accretes from the wind of a red giant .
The HD 154791 radial velocity variations can be fit with an eccentric orbit with period 404 \pm 3 d , amplitude K = 0.75 \pm 0.12 { km s ^ { -1 } } and eccentricity e = 0.26 \pm 0.15 .
There are also indications of variations on longer time scales \gtrsim 2000 d. We have re-examined all available ASM data following an unusually large X-ray outburst in 1997–98 , and confirm that the 1-d averaged 2–10 keV X-ray flux from 4U 1700+24 is modulated with a period of 400 \pm 20 d. The mean profile of the persistent X-ray variations was approximately sinusoidal , with an amplitude of 0.108 \pm 0.012 ASM { count s ^ { -1 } } ( corresponding to 31 % rms ) .
The epoch of X-ray maximum was approximately 40 d after the time of periastron according to the eccentric orbital fit .
If the 400-d oscillations from HD 154791/4U 1700+24 are due to orbital motion , then the system parameters are probably close to those of the only other neutron-star symbiotic-like binary , GX 1+4 .
We discuss the similarities and differences between these two systems .