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Within the context of the "B fields in OB stars (BOB)" collaboration, we usedthe HARPSpol spectropolarimeter to observe the early B-type stars beta CMa(HD44743; B1 II/III) and epsilon CMa (HD52089; B1.5 II). For both stars, weconsistently detected the signature of a weak (<30 G in absolute value)longitudinal magnetic field. We determined the physical parameters of bothstars and characterise their X-ray spectrum. For beta CMa, our modeidentification analysis led to determining a rotation period of 13.6+/-1.2 daysand of an inclination angle of the rotation axis of 57.6+/-1.7 degrees, withrespect to the line of sight. On the basis of these measurements and assuming adipolar field geometry, we derived a best fitting obliquity of ~22 degrees anda dipolar magnetic field strength (Bd) of ~100 G (60<Bd<230 G within 1 sigma),below what is typically found for other magnetic massive stars. For epsilon CMawe could only determine a lower limit on the dipolar magnetic field strength of13 G. For this star, we determine that the rotation period ranges between 1.3and 24 days. Both stars are expected to have a dynamical magnetosphere. We alsoconclude that both stars are most likely core hydrogen burning and that theyhave spent more than 2/3 of their main sequence lifetime. A histogram of thedistribution of the dipolar magnetic field strength for the magnetic massivestars known to date does not show the magnetic field "desert" observed insteadfor intermediate-mass stars. The biases involved in the detection of (weak)magnetic fields in massive stars with the currently available instrumentationand techniques imply that weak fields might be more common than currentlyobserved. Our results show that, if present, even relatively weak magneticfields are detectable in massive stars and that more observational effort isprobably still needed to properly access the magnetic field incidence.

B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type stars beta CMa and epsilon CMa