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We present the results of a new search for variable stars in the Local Groupdwarf galaxy Leo A, based on deep photometry from the Advanced Camera forSurveys onboard the Hubble Space Telescope. We detected 166 bona fide variablesin our field, of which about 60 percent are new discoveries, and 33 candidatevariables. Of the confirmed variables, we found 156 Cepheids, but only 10 RRLyrae stars despite nearly 100 percent completeness at the magnitude of thehorizontal branch. The RR Lyrae stars include 7 fundamental and 3first-overtone pulsators, with mean periods of 0.636 and 0.366 day,respectively. From their position on the period-luminosity (PL) diagram andlight-curve morphology, we classify 91, 58, and 4 Cepheids as fundamental,first-overtone, and second-overtone mode Classical Cepheids (CC), respectively,and two as population II Cepheids. However, due to the low metallicity of LeoA, about 90 percent of the detected Cepheids have periods shorter than 1.5days. Comparison with theoretical models indicate that some of the fainterstars classified as CC could be Anomalous Cepheids. We estimate the distance toLeo A using the tip of the RGB (TRGB) and various methods based on thephotometric and pulsational properties of the Cepheids and RR Lyrae stars. Thedistances obtained with the TRGB and RR Lyrae stars agree well with each otherwhile that from the Cepheid PL relations is somewhat larger, which may indicatea mild metallicity effect on the luminosity of the short-period Cepheids. Dueto its very low metallicity, Leo A thus serves as a valuable calibrator of themetallicity dependencies of the variable star luminosities.

The ACS LCID Project. VIII. The short-period Cepheids of Leo A