Radially pulsating variable stars found in the instability strip on the H-R Diagram have been known for

over a century now. For these stars, the principle mechanism causing pulsation has also been

understood for many decades. There remains, however, two pulsation characteristics that have never

been fully explained. These are the well-studied, Blazhko Effect and the period shifting variable stars.

The former effect causes periodic changes to the maximum brightness of the variable, while the later

causes the pulsation period to increase and decrease in a random fashion. Period shifting has not been

studied in detail to date and is often assumed to be a by-product of the Blazhko Effect. The period

shifting effect has been seen often in large variable star surveys where the period shifting occurs for

anywhere between 10 and 30% of the variables survived.

 

We have begun a program to explore the period shifting effect in RR Lyrae variables by investigating

the properties of a well known, bright, period shifting variable, RU Psc. Observations over the course

of nine years show random period shifts that amount to variations as large as 30 seconds/cycle. This

data also shows that RU Psc has temporal delays in its maximum light (up to 30 minutes) and a small

flicker in the maximum brightness which amounts to a variation of about 1.5%. High precision

observations conducted at the MacAdam Student Observatory during the Fall 2018, show a strong

correlation between the size of the rising compression hump and the delay in maximum light. This

suggests that the period shifting is likely related to intermittent shock waves that periodically occur

during the maximum compression phase. I will also discuss the differences between the shocks found

in Blazhko stars and that of RU Psc, which indicates that a mechanism, different from that of the

Blazhko Effect, is likely the cause of the period shifting phenomenon.