Two-epoch Zeeman–Doppler imaging of the close binary ER Vulpeculae

We present simultaneous magnetic reconstructions of both components of ER Vulpeculae from spectropolarimetric observationsobtained in 2013 and 2015. An SB2 extension of a Zeeman–Doppler imaging framework fits the two stars together using a spherical-harmonicexpansion through degree six, processed Stokes I profiles and observed Stokes V , with a reduced treatment of binary surface distortion. Bothcomponents exhibit the same directions of change in the reconstructed harmonic energy distribution. The toroidal fraction rises from 13.73%to 33.09% in component A and from 14.68% to 37.18% in component B; the dipole fraction decreases by 14.74 and 13.76 percentage points,respectively. The corresponding mean reconstructed fields are approximately 36/33 G in 2013 and 97/90 G in 2015. Known-field recovery testsreveal substantial topology bias in some noisy cases, and the saved fixed-mean refinements are limited by their iteration budgets. We thereforereport the parallel changes as properties of these reconstructions, while distinguishing them from a uniquely determined evolutionary signal orevidence for intercomponent dynamo coupling.