  We report deep EVN and eMERLIN observations of the Type Ia SN 2014J in the
nearby galaxy M 82. Our observations represent, together with JVLA observations
of SNe 2011fe and 2014J, the most sensitive radio studies of Type Ia SNe ever.
By combining data and a proper modeling of the radio emission, we constrain the
mass-loss rate from the progenitor system of SN 2014J to $\dot{M} \lesssim
7.0\times 10^{-10}\, {\rm M_{\odot}\, yr^{-1}}$ (3-$\sigma$; for a wind speed
of $100\, {\rm km s^{-1}}$). If the medium around the supernova is uniform,
then $n_{\rm ISM} \lesssim 1.3 {\rm cm^3}$ (3-$\sigma$), which is the most
stringent limit for the (uniform) density around a Type Ia SN. Our deep upper
limits favor a double-degenerate (DD) scenario--involving two WD stars--for the
progenitor system of SN 2014J, as such systems have less circumstellar gas than
our upper limits. By contrast, most single-degenerate (SD) scenarios, i.e., the
wide family of progenitor systems where a red giant, main-sequence, or
sub-giant star donates mass to a exploding WD, are ruled out by our
observations. Our estimates on the limits to the gas density surrounding SN
2011fe, using the flux density limits from Chomiuk et al. (2012), agree well
with their results. Although we discuss possibilities for a SD scenario to pass
observational tests, as well as uncertainties in the modeling of the radio
emission, the evidence from SNe 2011fe and 2014J points in the direction of a
DD scenario for both.
