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Using the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope,we have obtained the first time-resolved spectra of the King et al.ultraviolet-bright counterpart to the 11-minute binary X-ray source in the coreof the globular cluster NGC 6624. This object cannot be readily observed in thevisible, even from HST, due to a much brighter star superposed <0.1'' distant.Our FOS data show a highly statistically significant UV flux modulation with aperiod of 11.46+-0.04 min, very similar to the 685 sec period of the knownX-ray modulation, definitively confirming the association between the King etal. UV counterpart and the intense X-ray source. The UV amplitude is very largecompared with the observed X-ray oscillations: X-ray variations are generallyreported as 2-3% peak-to-peak, whereas our data show an amplitude of about 16%in the 126-251 nm range. A model for the system by Arons & King predictsperiodic UV fluctuations in this shortest-known period binary system, due tothe cyclically changing aspect of the X-ray heated face of the secondary star(perhaps a very low mass helium degenerate). However, prior to ourobservations, this predicted modulation has not been detected. Employing theArons & King formalism, which invokes a number of different physicalassumptions, we infer a system orbital inclination 35deg<i<50 deg. Amongst thethree best-studied UV/optical counterparts to the intense globular clusterX-ray sources, two are now thought to consist of exotic double-degenerateultrashort period binary systems.Comment: 10 pages including 2 figures in Latex (AASTeX 4.0). Accepted for  publication in vol. 482 (1997 June 10 issue) of The Astrophysical Journal  (Letters

Time-Resolved Ultraviolet Observations of the Globular Cluster X-Ray Source in NGC 6624: The Shortest Known Period Binary System