Validity of RR Lyrae epochs

Dear Forum Members,

I have another question concerning my Master's thesis project at Lund Observatory. It's about evaluating the how useful some old RR Lyrae epochs are. I hope the photometry forum is suitable for such a question.

I have spectra of two RR Lyrae stars in M 22, and I need to calculate their phases at the time when my spectra of them were obtained. The epochs I have are from Wehlau & Hogg 1978 (AJ vol. 83 no. 8), and they are from mid-1975. I know that the evolutionary changes of the period of a RR Lyrae star is quite slow (even on a scale of 10^6 years), but could something else make epochs invalid as time passes? Can the O-C diagram be used to estimate such things?

Kind Regards

Anders Nyholm

Lund, Sweden