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 GP And is a fast pulsating variable star with a period of 113.30 minutes and which involves a delta mag of 0.60 (R) magnitudes during this brief time interval. GP And is a delta Scuti variable and whose variable nature was discovered by W. Strohmeier, R. Kippenhahn and E.Geyer in 1956. GP And is a particularly well-studied variable star due to some studies (ex. Gieseking et al, 1979) which have suggested both a variable amplitude and a variable shape for the light curve. Further details from the International Variable Star Index are available here whereas an AAVSO finder chart is available here.