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We report a centimetre-wave (cm-wave, 5-31GHz) excess over free-free emission in PNe. Accurate 31 and 250GHz measurements show that the 31GHz flux densities in our sample are systematically higher than the level of optically thin free-free continuum extrapolated from 250GHz. The 31GHz excess is observed, within one standard deviation, in all 18 PNe with reliable 31 and 250GHz data, and is significant in 9 PNe. The only exception is the peculiar object M2-9, 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 31GHz, with a scatter of 11%. The average 31-250GHz spectral index of our sample is <alpha_{31}^{250}> = -0.43+-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 5GHz 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 cannot 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 31GHz, and for 26 objects at 250GHz.
This paper has been withdrawn by the author(s), as it is superseded by 0708.2385.
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