We report a centimetre-wave ( cm-wave , 5–31 GHz ) excess over free-free emission in PNe .
Accurate 31 and 250 GHz measurements show that the 31 GHz flux densities in our sample are systematically higher than the level of optically thin free-free continuum extrapolated from 250 GHz .
The 31 GHz excess is observed , within one standard deviation , in all 18 PNe with reliable 31 and 250 GHz data , and is significant in 9 PNe .
The only exception is the peculiar object M 2-9 , whose radio spectrum is that of an optically thick stellar wind .
On average the fraction of non-free-free emission represents 51 % of the total flux density at 31 GHz , with a scatter of 11 % .
The average 31–250 GHz spectral index of our sample is \langle \alpha _ { 31 } ^ { 250 } \rangle = -0.43 \pm 0.03 ( in flux density , with a scatter of 0.14 ) .
The 31–250 GHz drop is reminiscent of the anomalous foreground observed in the diffuse ISM by CMB anisotropy experiments .
The 5–31 GHz spectral indices are consistent with both flat spectra and spinning dust emissivities , given the 10 % calibration uncertainty of the comparison 5 GHz data .
But a detailed study of the objects with the largest cm-excess , including the low frequency data available in the literature , shows that present spinning dust models can not alone explain the cm-excess in PNe .
Although we have no definitive interpretation of our data , the least implausible explanation involves a synchrotron component absorbed by a cold nebular screen .
We give flux densities for 37 objects at 31 GHz , and for 26 objects at 250 GHz .