We report on VLBI measurements of supernova 2014C at several epochs between t = 384 and 1057 days after the explosion .
SN 2014C was an unusual supernova that initially had Type Ib optical spectrum , but after t = 130 d it developed a Type IIn spectrum with prominent H \alpha lines , suggesting the onset of strong circumstellar interaction .
Our first VLBI observation was at t = 384 d , and we find that the outer radius of SN 2014C was ( 6.40 \pm 0.26 ) \times 10 ^ { 16 } cm ( for a distance of 15.1 Mpc ) , implying an average expansion velocity of 19300 \pm 790 km s ^ { -1 } up to that time .
At our last epoch , SN 2014C was moderately resolved and shows an approximately circular outline but with an enhancement of the brightness on the W side .
The outer radius of the radio emission at t = 1057 d is ( 14.9 \pm 0.6 ) \times 10 ^ { 16 } cm .
We find that the expansion between t = 384 and 1057 d is well described by a constant velocity expansion with v = 13600 \pm 650 km s ^ { -1 } .
SN 2014C had clearly been substantially decelerated by t = 384 d. Our measurements are compatible with a scenario where the expanding shock impacted upon a shell of dense circumstellar material during the first year , as suggested by the observations at other wavelengths , but had progressed through the dense shell by the time of the VLBI observations .