  Chemically peculiar stars of the mercury-manganese (HgMn) type represent a
new class of spotted late-B stars, in which evolving surface chemical
inhomogeneities are apparently unrelated to the presence of strong magnetic
fields but are produced by some hitherto unknown astrophysical mechanism. The
goal of this study is to perform a detailed line profile variability analysis
and carry out a sensitive magnetic field search for one of the brightest HgMn
stars - mu Lep. We acquired a set of very high-quality intensity and
polarization spectra of mu Lep with the HARPSpol polarimeter. These data were
analyzed with the multiline technique of least-squares deconvolution in order
to extract information on the magnetic field and line profile variability. Our
spectra show very weak but definite variability in the lines of Sc, all Fe-peak
elements represented in the spectrum of mu Lep, as well as Y, Sr, and Hg.
Variability might also be present in the lines of Si and Mg. Anomalous profile
shapes of Ti II and Y II lines suggest a dominant axisymmetric distribution of
these elements. At the same time, we found no evidence of the magnetic field in
mu Lep, with the 3 sigma upper limit of only 3 G for the mean longitudinal
magnetic field. This is the most stringent upper limit on the possible magnetic
field derived for a spotted HgMn star. The very weak variability detected for
many elements in the spectrum mu Lep suggests that low-contrast chemical
inhomogeneities may be common in HgMn stars and that they have not been
recognized until now due to the limited precision of previous spectroscopic
observations and a lack of time-series data. The null result of the magnetic
field search reinforces the conclusion that formation of chemical spots in HgMn
stars is not magnetically driven.
