This is a review article which elaborates the results presented in [1], where
the variational principle of Herglotz type with a Lagrangian that depends on
fractional derivatives of both real and complex orders is formulated and the
invariance of this principle under the action of a local group of symmetries
is determined. The conservation law for the corresponding fractional Euler
Lagrange equation is obtained and a sequence of approximations of a
fractional Euler-Lagrange equation by systems of integer order equations
established and analyzed.