Observational Constraints on the Magnetic Field of RR Lyrae Stars

In some of the models for explaining the Blazhko effect, the modulation is caused by the effects of a magnetic field. We carried out a survey to determine whether RR Lyrae stars are generally characterized by the presence of a magnetic field organized on a large scale. With the help of the FORS1 instrument at the ESO VLT we obtained spectropolarimetric observations of 17 relatively bright southern RR Lyrae stars, both Blazhko stars and non-modulated stars, and determined their mean longitudinal magnetic field with a typical error bar < 30 G. From our data we can set an upper limit for the strength of the dipole component of the magnetic fields of the RR Lyrae stars in our sample. The outcome of our survey reveals that the Blazhko modulation in the pulsation of RR Lyrae stars is not correlated with the presence of a strong, quasi-dipolar magnetic field.