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An extremely weak circularly-polarized signature was recently discovered inspectral 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-gausslongitudinal magnetic field, constituting the first detection of a surfacemagnetic field in an Am star. We present here ultra-deep spectropolarimetricobservations of two other bright Am stars, $\beta$ UMa and $\theta$ Leo,observed with the NARVAL spectropolarimeter. The line profiles of the two starsdisplay circularly-polarized signatures similar in shape to the observationsgathered for Sirius A. These new detections suggest that very weak magneticfields may be present in the photospheres of a significant fraction ofintermediate-mass stars, although the strongly asymmetric Zeeman signaturesmeasured so far in Am stars (featuring a prominent positive lobe and nodetected negative lobe) are not expected in the standard theory of the Zeemaneffect.

Ultra weak magnetic fields in Am stars: Beta UMa and theta Leo