Spectropolarimetric study of the classical Cepheid $η$ Aql: pulsation and magnetic field

We report the results of spectropolarimetric study of the classical Cepheid $\eta$ Aql. We found that the longitudinal magnetic field of $\eta$ Aql sinusoidally varies with the radial pulsation period, while the amplitude $B$, mean field $B_0$, and phases of maximum and minimum field change from year to year. We hypothesize that possible reasons of those variations are stellar axial rotation or dynamo mechanisms.