  We have used the Advanced Camera for Surveys on board the Hubble Space
Telescope to image the core of the globular cluster M15 in the far-ultraviolet
(FUV) waveband. Based on these observations, we identify the FUV counterpart of
the recently discovered low-mass X-ray binary M15 X-2. Our time-resolved FUV
photometry shows a modulation with 0.062+/-0.004 mag semi-amplitude and we
clearly detect a period of 22.5806+/-0.0002 min. We have carried out extensive
Monte Carlo simulations which show that the signal is consistent with being
coherent over the entire observational time range of more than 3000 cycles.
This strongly suggests that it represents the orbital period of the binary
system. M15 X-2 is FUV bright (approx. 17 mag) and is characterized by an
extremely blue spectral energy distribution (F_lambda ~ lambda^{-2.0}). We also
find evidence for an excess of flux between 1500 and 1600 \AA and probably
between 1600 and 2000 \AA, which might be due to CIV 1550 and HeII 1640
emission lines. We also show that M15 X-2's X-ray luminosity can be powered by
accretion at the rate expected for gravitational-wave-driven mass transfer at
this binary period. The observed FUV emission appears to be dominated by an
irradiated accretion disk around the neutron star primary, and the variability
can be explained by irradiation of the low-mass white dwarf donor if the
inclination of the system is approx. 34 degree. We conclude that all
observational characteristics of M15 X-2 are consistent with it being an
ultracompact X-ray binary, only the third confirmed such object in a globular
cluster.
