  A detailed zero and transverse field (ZF&TF) muon spin rotation (muSR)
investigation of magnetism and the magneto-electric coupling in Cu2OSeO3 is
reported. An internal magnetic field B_int(T=0) = 85.37(25) mT was found, in
agreement with a ferrimagnetic state below Tc = 57.0(1) K. The temperature
dependence of the magnetic order parameter is well described by the relation
B_int = B(0)(1-(T/Tc)^2)^b with an effective exponent b = 0.39(1) which is
close to the critical exponent B ~ 1/3 for a three dimensional (3D) magnetic
system. Just above Tc the muon relaxation rate follows the power low \lambda
(T)\propto (T/Tc - 1)^\omega with \omega = 1.06(9), which is characteristic for
3D ferromagnets. Measurements of B_int(T) with and without an applied
electrostatic field E = 1.66 x 10^5 V/m suggest a possible electric field
effect of magnitude \Delta Bv = Bv(0 V)-Bv(500 V) = - 0.4(4) mT.
