We present the detection of a variable star with an 18.05 minute period in far-ultraviolet ( FUV ) images of the globular cluster NGC 1851 taken with the Hubble Space Telescope ( HST ) .
A candidate optical counterpart lies on the red horizontal branch or the asymptotic giant branch star of the cluster , but it is statistically possible that this is a chance superposition .
This interpretation is supported by optical spectroscopt obtained with HST/STIS : the spectrum contains none of the strong emission lines that would be expected if the object was a symbiotic star ( i.e .
a compact accretor fed by a giant donor ) .
We therefore consider two other possibilities for the nature of FUV variable : ( i ) an intermediate polar ( i.e .
a compact binary containing an accreting magnetic white dwarf ) , or ( ii ) an AM CVn star ( i.e .
an interacting double-degenerate system ) .
In the intermediate polar scenario , the object is expected to be an X-ray source .
However , no X-rays are detected at its location in \simeq 65 ksec of Chandra imaging , which limits the X-ray luminosity to L _ { X } \leqslant 10 ^ { 32 } erg s ^ { -1 } .
We therefore favour the AM CVn interpretation , but a FUV spectrum is needed to distinguish conclusively between the two possibilities .
If the object is an AM CVn binary , it would be the first such system known in any globular cluster .