Time-Frequency Analysis and Pulsation Modes of LPV Stars.I. omicron Ceti

The problem of the pulsation modes of the Long Period Variable star o Cet is investigated by means of Fourier and wavelet transforms applied to long-term AAVSO visual observations. The results suggest that the variability of this star is mainly due to first overtone radial pulsation, nonlinearly coupled to at least two other radial modes. This is supported by linear, nonadiabatic modelling, though fundamental-mode or subharmonic pulsation cannot be definitely ruled out. Theoretical estimates of the fundamental parameters of the star are derived from the periods of the modes. The luminosity and the effective temperature are found in agreement with the best recent observational estimates. These results, and the discussion of recent nonlinear calculations, suggest that linear modes might be basically preserved in the nonlinear pulsation, though with a moderate period shift.