Distributions of the circular polarization degree, m_c, measurements at 4.85 GHz (top) and 8.35 GHz (bottom) for three observing sessions (solid black, dashed red and dotted blue lines). The non-zero average values, shown in the legend, indicate the presence of instrumental circular polarization. The m_c measurements of the planetary nebula NGC 7027, in the corresponding sessions, are marked with arrows (in the bottom plot, two arrows overlap).