The Collaborative International Dictionary of English v.0.48:

Corroboratory \Cor*rob"o*ra*to*ry\ (-t?-r?), a.
   Tending to strengthen; corroborative; as, corroboratory
   facts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

corroboratory
    adj 1: serving to support or corroborate; "collateral evidence"
           [syn: collateral, confirmative, confirming,
           confirmatory, corroborative, corroboratory,
           substantiating, substantiative, validating,
           validatory, verificatory, verifying]