A variable star, as its name suggests, is a star whose magnitude varies intrinsically, in contrast to eclipsing binaries whose magnitude varies as a result of one star in the binary system eclipsing the other. True variables are one of five types, namely Mira stars, semiregular stars, cepheids, eruptive variables and, finally, cataclysmic variables. Minimum to maximum magnitude can range from days to many months with some variables displaying irregular periods.

A popular method for the study of variable stars, particularly short-term variables, is by the use of the technique known as "differential photometry". Rather than measure the (variable) magnitude of a variable star on an absolute scale, measurements are made over time relative to one or more non-variable star(s) and these differences are then plotted so as to study and illustrate the relative or differential change in magnitude. Due to the very large number of variables stars, the field of differential photometry represents one of the key fields in astronomy whereby the amateur astronomer can make a meaningful and long-lasting contribution to both science and astronomy.

More recently, the search for extrasolar planets (over 750 discovered so far) has identified yet another interesting application for the practice of differential photometry whereby the minute drops in magnitude of a star hosting an exoplanet are studied. Further details for the interested party are available here.

Note: The eclipsing binary star V441 Lac is a quick eclipsing binary variable star with a period of 0.3089113 days and which involves a delta mag of 0.22 (r) and 0.19 (r) magnitudes for the primary and secondary minima, respectively, during this brief time interval. The variable nature of V441 Lac was discovered by Franz Agerer in 2001 and which happens to lie in the same field of view as the eclipsing binary IU Lac. V441 Lac is a W Ursae Majoris-type eclipsing system where the ellipsoidal components are virtually in contact with each other and, hence, for its classification, namely "EW". Further details from the International Variable Star Index are available here whereas an AVVSO finder chart is available here.

Note: The minima observed below represent the secondary and primary minima, respectively.