Phase lags in the optical-infrared light curves of asymptotic giant branch stars

To search for phase lags in the optical-infrared light curves of asymptotic giant branch stars, we have compared infrared data from the COBE DIRBE satellite with optical light curves from the AAVSO and other sources. We found 17 examples of phase lags between the times of infrared and optical maximum, and 4 stars with no observed lags. There is a clear difference between the Mira variables and the semiregular variables in the sample, with the maximum in the optical preceding that in the near-infrared in the Miras, while in most of the semiregular variables no lags are observed. Comparison to published theoretical models indicates that the phase lags in the Miras are due to strong titanium oxide absorption in the visual at stellar maximum, and suggests that Miras pulsate in the fundamental mode, while at least some semiregular variables are first-overtone pulsators. There is a clear optical-near-infrared phase lag in the carbon-rich Mira V CrB; this is likely due to C-2 and CN absorption variations in the optical.