  The classical van Royen-Weisskopf formula for the decay width of a meson into
a lepton-antilepton pair is modified in order to include non-zero quark
momentum contributions within the meson as well as relativistic effects.
Besides, a phenomenological electromagnetic density for quarks is introduced.
The meson wave functions are obtained from two different models: a chiral
constituent quark model and a quark potential model including instanton
effects. The modified van Royen-Weisskopf formula is found to improve
systematically the results for the widths, giving an overall good description
of all known decays.
