  We present a complete census of RR Lyrae stars in a halo field of the
Andromeda galaxy. These deep observations, taken as part of a program to
measure the star formation history in the halo, spanned a period of 41 days
with sampling on a variety of time scales, enabling the identification of short
and long period variables. Although the long period variables cannot be fully
characterized within the time span of this program, the enormous advance in
sensitivity provided by the Advanced Camera for Surveys on the Hubble Space
Telescope allows accurate characterization of the RR Lyrae population in this
field. We find 29 RRab stars with a mean period of 0.594 days, 25 RRc stars
with a mean period of 0.316 days, and 1 RRd star with a fundamental period of
0.473 days and a first overtone period of 0.353 days. These 55 RR Lyrae stars
imply a specific frequency S_RR=5.6, which is large given the high mean
metallicity of the halo, but not surprising given that these stars arise from
the old, metal-poor tail of the distribution. This old population in the
Andromeda halo cannot be clearly placed into one of the Oosterhoff types: the
ratio of RRc/RRabc stars is within the range seen in Oosterhoff II globular
clusters, the mean RRab period is in the gap between Oosterhoff types, and the
mean RRc period is in the range seen in Oosterhoff I globular clusters. The
periods of these RR Lyraes suggest a mean metallicity of [Fe/H]=-1.6, while
their brightness implies a distance modulus to Andromeda of 24.5+/-0.1, in good
agreement with the Cepheid distance.
