On the Swedish Variable Observations database, SVO, have I begun a project to publish O-C diagrams for Miras based on data from AAVSO and AFOEV. Togheter with the diagrams is some information about the stars from GCVS4, magnitude at max from the maxima found and the result from a linear and parabolic fit to the O-C diagram. A linear trend suggest that the period is constant, but wrong. A parabolic trend that the period is changing with a constant rate. The statiscally significance of the fit is also presented. You can also get a table of the individual maxima and a O-C diagram of the residuals when the linear or parabolic trend is removed.
The digrams can be found at http://var.astronet.se/mirainfooc.php [1] or by SVO's homepage at http://var.astronet.se [2] end then choosing "Ljuskurvor & Resultat -> O-C diagram för Miror". The O-C diagrams are in english but the rest of the site is in swedish...
Thomas Karlsson
Links:
[1] http://var.astronet.se/mirainfooc.php
[2] http://var.astronet.se
[3] http://www.aavso.org/forums/variable-stars/long-period-variables-lpvs