Robust Variable Star Detection Techniques Suitable for Automated searches: New Results for NGC 1866

We describe sensitive techniques for identifying variable stars in multiepoch photometry lists made in two or more bandpasses or pairwise in a single bandpass. Our search strategy employs the correlation in brightness change between paired frames, which allows the extraction of information on variability in the presence of random photometric noise. A statistic for assessing the tape of variable or degree of contamination by an unresolved source (depending on circumstance) is also described, as well as a bootstrap scheme for predicting confidence intervals for detection of real variability. These detection schemes are manifestly more robust than existing search techniques. To illustrate the power of this technique, we give a two-epoch example for photometry in the field of NGC 3201, a globular cluster rich in RR Lyr variables. Even this simple application has allowed us to rediscover 34 of 57 previously known (actual) variables, to verify the misidentification of several of them, and to discover four probable new variables (three of which may be RR Lyr stars). We then apply the variability statistics to 27 V and 29 B frames centered on the young, rich, Cepheid-bearing LMC cluster NGC 1866, which have been reduced with ALLFRAME. This application has resulted in the discovery of 12 new variables, as well as the recovery of all 15 of the previously known variables that lie within this field. Specific results are: (1) the discover of four new Cepheid variables in NGC 1866, bringing the total number of probable member Cepheids to 23, (2) periods for all but one of the 23 NGC 1866 Cepheids, (3) good evidence for photometric contamination of several variables by unresolved companions- (4) the discovery of a RRab variable in the NGC 1866 field with a peak-to-peak V light-curve amplitude of 1.0 mag, an intensity-weighted mean V mag = 19.28+/- 0.01 mag, and a period of 0.5565 days, (5) (re)discovery of three longer-period red variables, and (6) the detection of several main-sequence or main-sequence-turnoff variable stars.