We point out that the equivalent-photon approximation (EPA) for processes with massive spin-1 particles in the final state would have validity in a more restricted kinematic domain than for processes where it is commonly applied, viz., those with spin-1/2 or spin-0 particles in the final state. We obtain the criterion for the validity of EPA for the two-photon production of a pair of charged, massive, point-like spin-1 particles V±, each of mass M and with a standard magnetic moment (κ=1). In a process in which one of the photons is real and the other virtual with four-momentum q, the condition for the validity of EPA is |q2|≪M2, in addition to the usual condition |q2|≪W2, W being the V+ V- invariant mass. In a process in which both photons are virtual (with four-momenta q and q′), our condition is |q2
q′2|W4 ≪ 16 M8, in addition to |q2| ≪M2, |q′2| ≪M2 and |q2| ≪W2, |q′2| ≪W2. Even when these extra conditions permitting the use of EPA are not fulfilled, convenient approximate expressions may still be obtained assuming merely |q2| ≪W2 and |q′2| ≪W2. We also discuss how the extra conditions are altered when the vector bosons are incorporated in a spontaneously broken gauge theory. Examples of W boson production in Weinberg-Salam model are considered for which the condition |q2
q′2|W4 ≪ 16 M8 is shown to be removed. © 1986 the Indian Academy of Sciences.
