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 can not 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 } \approx 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 can not 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 ] \approx - 1.6 , while their brightness implies a distance modulus to Andromeda of 24.5 \pm 0.1 , in good agreement with the Cepheid distance .