The X-ray source 4U 1820 - 30 in the globular cluster NGC 6624 is known as the most compact binary among the identified X-ray binaries .
Having an orbital period of 685.0 s , the source consists of a neutron star primary and likely a 0.06–0.08 M _ { \sun } white dwarf secondary .
Here we report on far-ultraviolet ( FUV ) observations of this X-ray binary , made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope .
From our Fourier spectral analysis of the FUV timing data , we obtain a period of 693.5 \pm 1.3 s , which is significantly different from the orbital period .
The light curve folded at this period can be described by a sinusoid , with a fractional semiamplitude of 6.3 % and the phase zero ( maximum of the sinusoid ) at MJD 50886.015384 \pm 0.000043 ( TDB ) .
While the discovered FUV period may be consistent with a hierarchical triple system model that was previously considered for 4U 1820 - 30 , we suggest that it could instead be the indication of superhump modulation , which arises from an eccentric accretion disk in the binary .
The X-ray and FUV periods would be the orbital and superhump periods , respectively , indicating a 1 % superhump excess and a white-dwarf/neutron-star mass ratio around 0.06 .
Considering 4U 1820 - 30 as a superhump source , we discuss the implications .