The amplification of a light beam due to intervening gravitational waves is
studied. The previous Jordan frame result according to which the amplification
is many orders of magnitude larger in scalar-tensor gravity than in general
relativity does not hold in the Einstein conformal frame. Lensing by
gravitational waves is discussed in relation to the ongoing and proposed VLBI
observations aimed at detecting the scintillation effect.Comment: 12 pages, LaTeX, to appear in Astronomy & Astrophysic
