ROTSE all-sky surveys for variable stars. I. Test fields

The Robotic Optical Transient Search Experiment I(ROTSE-I) experiment has g enerated CCD photometry for the entire northern sky in two epochs nightly s ince 1998 March. These sky patrol data are a powerful resource for studies of astrophysical transients. As a demonstration project, we present first r esults of a search for periodic variable stars derived from ROTSE-I observa tions. Variable identification, period determination, and type classificati on are conducted via automatic algorithms. In a set of nine ROTSE-I sky pat rol fields covering roughly 2000 deg(2), we identify 1781 periodic variable stars with mean magnitudes between m(v) = 10.0 and m(v) = 15.5. About 90% of these objects are newly identified as variable. Examples of many familia r types are presented. All classifications for this study have been manuall y confirmed. The selection criteria for this analysis have been conservativ ely defined and are known to be biased against some variable classes. This preliminary study includes only 5.6% of the total ROTSE-I sky coverage, sug gesting that the full ROTSE-I variable catalog will include more than 32,00 0 periodic variable stars.