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 large-amplitude short-period pulsating star AU CrB is a fast pulsating variable star with a period of 74.4 minutes and which involves a delta mag of 0.23 (v) magnitudes during this brief time interval. AU CrB is a delta-Scuti variable star first suspected to be variable by Akerlof et al. in 2000 during inspection of the Robotic Optical Transient Search Experiment I (ROTSE-I) database involving the entire northern sky. In a follow-up paper by Wils et al, AU CrB was described as having a period much shorter than that presented by Akerlof and for this variable to be a double-mode radial pulsator with a primary frequency at 19.335 c/d and a secondary frequency at 25.005 c/d, thus corresponding to periods of 0.0517 days and 0.0400 days, respectively. Further details from the International Variable Star Index are available here whereas an AAVSO finder chart is available here.