AstroImageJ exercise -- what is the period of a variable star?

We will measure the brightness of these three stars. Be sure when using the Analyze -> Multi-aperture tool that you click first on star "A", then "B", and finally "C". If your image doesn't have the same orientation as the chart below, try View -> Invert Y.

The images we will analyze are as follows:

Group 1 starts at image 20, measures it and the next three (25, 30, 35)
Group 2 starts at image 30, measures it and the next three (35, 40, 45)
Group 3 starts at image 40, measures it and the next three (45, 50, 55)
...
Group 20 starts at image 205, measures it and the next three (210, 215, 220)

Group 21 starts at image 25, measures it and the next three (30, 35, 40)
Group 22 starts at image 35, measures it and the next three (40, 45, 50)
Group 23 starts at image 45, measures it and the next three (40, 45, 50)
...


		  asas14clclear-020.fit
		  asas14clclear-025.fit
		  asas14clclear-030.fit
		  asas14clclear-035.fit
		  asas14clclear-040.fit
		  asas14clclear-045.fit
		  asas14clclear-050.fit
		  asas14clclear-055.fit
		  asas14clclear-060.fit
		  asas14clclear-065.fit
		  asas14clclear-070.fit
		  asas14clclear-075.fit
		  asas14clclear-080.fit
		  asas14clclear-085.fit
		  asas14clclear-090.fit
		  asas14clclear-095.fit
		  asas14clclear-100.fit
		  asas14clclear-105.fit
		  asas14clclear-110.fit
		  asas14clclear-115.fit
		  asas14clclear-120.fit
		  asas14clclear-125.fit
		  asas14clclear-130.fit
		  asas14clclear-135.fit
		  asas14clclear-140.fit
		  asas14clclear-145.fit
		  asas14clclear-150.fit
		  asas14clclear-155.fit
		  asas14clclear-160.fit
		  asas14clclear-165.fit
		  asas14clclear-170.fit
		  asas14clclear-175.fit
		  asas14clclear-180.fit
		  asas14clclear-185.fit
		  asas14clclear-190.fit
		  asas14clclear-195.fit
		  asas14clclear-200.fit
		  asas14clclear-205.fit
		  asas14clclear-210.fit
		  asas14clclear-215.fit
		  asas14clclear-220.fit

The star we are measuring is an example of a cataclysmic variable system. This consists of two stars orbiting around each other in a tight binary. One star is an "ordinary" star, not unlike our Sun, but the other is a very compact, dense, hot star called a "white dwarf."

Image courtesy of David Hardy and PPARC.