  We have searched for variable sources in the core region of M80, using far
ultra-violet data taken with the Advanced Camera for Surveys on board the
Hubble Space Telescope. We found three sources that exhibit strong signs of
variability in our data. Among these is source TDK1, which we believe to be an
RR Lyrae star that reached maximum brightness during our observations. The
light curve shows a >3 mag FUV brightening over the course of ~5 hours, with an
estimated peak brightness of ~16.7 mag, followed by a decrease to ~20 mag.
Archival optical data obtained with WFPC2 confirm that TDK1 is variable in all
wavebands. TDK1's SED is reasonably fit by a star with temperature T(eff)=6700K
and radius R=4.2R(sun), consistent with the suggestion that it is an RR Lyrae.
Based on the photometric and variability characteristics of the other two
variables, we suggest that TDK2 is likely to be an SX Phoenicis star with ~55
minutes period, and TDK3 is likely another RR Lyrae. Finally, we briefly
discuss the FUV counterparts to two previously known variables in M80, the
classical nova T Sco and the dwarf nova DN1.
