2002A&A...391..633P

The changes in the periods of the fundamental and first overtone pulsation of the variable V26 in the globular cluster M15 are studied. The analysis was based on published observations. The method of investigation was outlined in our previous paper (Paparo et al., 1998A&A...332..102P). The Fourier phase diagrams for both frequencies have been constructed. General guidelines concerning the cycle counting problems are discussed. In both periods a sudden increase took place around JD 2431500 (≃1945) and the changes were the same in sign but slightly different in magnitude. The period change behaviour of the fundamental mode was noticed for the first time. Before the abrupt change the fundamental and first overtone periods were P₀=0.538898d and P₁=0.402256d days and after it P₀=0.538979d and P₁=0.402273d days. Although the fundamental component is very weak, there is a remarkable concordance between our period after the abrupt change and the period derived by Clement & Walker (1991AJ....101.1352C). The disagreement between the observed period changes and the expectation of stellar evolutionary theory strongly suggest that period changes are dominated by short term changes governed by the physics of the star. Fast changes in oscillation frequency such as in V26/M15 cannot take place without sudden changes in the stellar structure. The possible physical processes are the mixing events in a small part of the semiconvective zone (Sweigart & Renzini, 1979A&A....71...66S) or the dredge up of a small amount of helium by occasional convection overshooting (Cox, 1998ApJ...496..246C).