  We present a special-relativistic analysis of deformable interferometers
where counter propagating beams share a common optical path. The optical path
is allowed to change rather arbitrarily and need not be stationary. We show
that, in the absence of dispersion the phase shift has two contributions. To
leading order in $v/c$ one contribution is given by Wang empirical formula for
deformable Sagnac interferometers. The second contribution is due to the
stretching of the optical path and we give an explicit formula for this stretch
term valid to first order in $v/c$. The analysis provides a unifying framework
incorporating the Sagnac, Wang and Fizeau effects in a single scheme and gives
a rigorous proof of Wang empirical formula.
