Studies of the Large Magellanic Cloud. X. Photometry of variable stars.

Using three photoelectric sequences obtained at the Mount Stromlo Observatory, we have studied the variable stars of a section of the Large Magellanic Cloud centered at a = 5b12 = -67?5. Measures in two colors, B and V, have been obtained on a series of eighty ADH Schnddt plates, and measures in the blue were made on approximately four hundred Harvard Bruce plates for each variable star. Because of the long time interval of the plates, highiy accurate periods of the Cepheids could be obtained, to about six significant figures in most cases. No period changes were detected that could not be attributed to data limitations. The period4uminosity relation has the form (B) = 18.28 - 2.80 log P, (V) = 17.82 - 3.05 log P, uncorrected for reddening. The slope agrees well with previous investigations if sampling problems and the probable curvature of the relation are taken into account. There is a significant zero- point difference with the results of Woolley et al. that can only be explained in terms of photometric differences. Agreement with two recent comprehensive redeterminations of the period4uminosity relation is excellent, and these lead to distance moduli for the Cloud averaging 18.90, corrected for reddening. Some differences in period-amplitude relations are found for the LMC, SMC, and our Galaxy, and these are not likely to be due wholly to selection effects. The period-color relation for LMC Cepheids agrees well with that for our Galaxy, much better than reported in previous studies. Two W Virginis variables are included, each with changing period and each subluminous for its period in comparison with the classical Cepheids. Two long-period variables, one of rather short (161 days) and one of unusually long (806 days) period, agree with the apparent period-maximum magnitude relation found for SMC long-period variables. An R CrB star with a single observed minimum of 2300 days was detected. Two p eclipsing variables complete the list. A comprehensive color-magnitude diagram illustrates the relationship between all of these variable stars and the non-variable stars of the region.