9. a) The period is approximately 30 days and the amplitude
is 1.5 units.
b) Day 80 is 20 days into the pattern because
80 = 2(30) + 20. On day 20 the Luminosity number is
approximately 4.67 units. Day 100 is 10 days into the
pattern because 100 = 3(30) + 10. On day 10 the
Luminosity number is approximately 2.74 units.
c) This is a star system where two stars are orbiting each
other. It appears that one of the stars is brighter than
the other one. The Luminosity number decreases
sharply each time the less bright star passes in front of
(eclipses) the brighter star and decreases by a lesser
amount when the brighter star eclipses the less bright
star. The Luminosity number is higher when neither of
the stars is eclipsed by the other one.

Exercise 4.2