We report the late-time evolution of Type IIb Supernova ( SN IIb ) 2013df .
SN 2013df showed a dramatic change in its spectral features at \sim 1 year after the explosion .
Early on it showed typical characteristics shared by SNe IIb/Ib/Ic dominated by metal emission lines , while later on it was dominated by broad and flat-topped H \alpha and He I emissions .
The late-time spectra are strikingly similar to SN IIb 1993J , which is the only previous example clearly showing the same transition .
This late-time evolution is fully explained by a change in the energy input from the ^ { 56 } Co decay to the interaction between the SN ejecta and dense circumstellar matter ( CSM ) .
The mass loss rate is derived to be \sim ( 5.4 \pm 3.2 ) \times 10 ^ { -5 } M _ { \odot } yr ^ { -1 } ( for the wind velocity of \sim 20 km s ^ { -1 } ) , similar to SN 1993J but larger than SN IIb 2011dh by an order of magnitude .
The striking similarity between SNe 2013df and 1993J in the ( candidate ) progenitors and the CSM environments , and the contrast in these natures to SN 2011dh , infer that there is a link between the natures of the progenitor and the mass loss : SNe IIb with a more extended progenitor have experienced a much stronger mass loss in the final centuries toward the explosion .
It might indicate that SNe IIb from a more extended progenitor are the explosions during a strong binary interaction phase , while those from a less extended progenitor have a delay between the strong binary interaction and the explosion .