We present preliminary results concerning the search for short-period variable stars in Tucana and LGS3 based on very deep HST/ACS imaging. In the one chip per galaxy we studied so far, a total of 230 and 80 candidates variables were found, respectively. For Tucana, we identified 134 of them as RR Lyrae stars (RRL) pulsating in the fundamental mode (RRab), 51 in the first-overtone mode (RRc), and 37 in both modes simultaneoulsy (RRd), as well as four candidate anomalous Cepheids (AC). In the case of LGS3, we found 45 RRab and 5 RRc, plus three candidates RRd and five candidate AC. The metallicities obtained from the mean period of the RRab are [Fe/H]Tuc = − 1.7 and [Fe/H]LGS3 = − 1.8.