There is observational evidence that intermittent long secondary periods of ~1000?days are present in the well-observed yellow hypergiants ρ Cas and HR 8752. The long secondary period is interpreted here as the turnover time of giant convection cells in the convective envelope, as has been already suggested in the case of red giants and supergiants of high luminosity. The observed secondary periods and surface radial velocities of ρ Cas and HR 8752 agree with the theoretical predictions, within the expected errors. These results support a theoretical interpretation that now covers the entire initial mass range from 1 to 50 M ☉ for luminous cool stars.
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