S Serpentis is a fine example of a Mira variable with a bump in its light curve. There are a number of Miras that show cycle-to-cycle variations in their light curves, and such variability is more common than not among the Mira class of stars. The reasons why bumps or secondary peaks occur aren't fully understood yet, but could be related to the occurrence of secondary shocks in the atmosphere. The greatly extended and tenuous nature of Mira photospheres and atmospheres makes them a challenge to model, but light curves like those of the AAVSO help provide useful data that models can be compared to.
Prepared By: Matthew Templeton