  Following Polchinski's approach we calculate the one-loop vacuum amplitude
for two parallel D-branes connected by open bosonic (neutral or charged)string
in a constant uniform electromagnetic (EM) field. For neutral string, external
EM field contribution appears as multiplier (Born-Infeld type action) of vacuum
amplitude without external EM field. Hence,it gives the alternative way to see
the inducing of Born-Infeld type action for description of D-branes. For
charged string the situation is more complicated, it may indicate the necessity
to modify the induced D-branes action in this case.
