Multi-frequency observations of K3-9 - a new radio-luminous symbiotic Mira

K3-9 ranks among the most intense symbiotic radio sources, and most of those that are brighter have experienced very slow nova outbursts within the last century. The intruiging combination of extreme visual faintness and high radio flux density led us to obtain multi-frequency coverage in order to determine physical parameters of the source. We report measurements of K3-9 covering five orders of magnitude in wavelength, including the first IR, millimetre and radio data. Near-IR spectra and photometry clearly demonstrate that the system contains a late-type giant (a Mira or OH/IR star such as those found in D-type symbiotics) which is probably shrouded in a dusty circumbinary envelope. This reconciles the level of radio and IR emission with the visual magnitude and the high Balmer decrement. The radio spectrum is similar to that of other symbiotic Miras and is best represented by a single power law with spectral index alpha=0.81+/-0.02 it flattens at a frequency of approximately 8 GHz, before steepening again beyond 90 GHz. This submillimetre flux excess is probably due to emission from dust (possibly material lost by the hot star, or by one or more symbiotic nova outbursts) and it is reminiscent of R Aqr which also has its spectral turnover hidden by dust emission. The implied dust mass is 0.01-0.15 M_solar for 30<T_d<300 K, respectively. The ionized region in K3-9 has been slightly resolved (FWHM~0.1 arcsec~750au) by the A configuration of the Very Large Array at 8.44 GHz and by the BnA configuration at 22.5 GHz. We have determined the systemic distance and made model-dependent estimates of the Mira's mass-loss rate, the binary separation and the Lyman continuum luminosity.