Ultra-weak magnetic fields in Am stars: β UMa and θ Leo

An extremely weak circularly-polarized signature was recently discovered in spectral lines of the chemically peculiar Am star Sirius A (Petit et al. 2011). This signal was interpreted as a Zeeman signature related to a sub-gauss longitudinal magnetic eld, constituting the rst detection of a surface magnetic eld in an Am star. We present here ultra-deep spectropolarimetric observations of two other bright Am stars, UMa and Leo, observed with the NARVAL spectropolarimeter. The line proles of the two stars display circularly-polarized signatures similar in shape to the observations gathered for Sirius A. These new detections suggest that very weak magnetic elds may be present in the photospheres of a signicant fraction of intermediate-mass stars, although the strongly asymmetric Zeeman signatures measured so far in Am stars (featuring a prominent positive lobe and no detected negative lobe) are not expected in the standard theory of the Zeeman eect.