  We present a general framework for the homogenisation theory of space-time
metamaterials. By mapping to a frame co-moving with the space-time modulation,
we derive analytical formulae for the effective material parameters for
travelling wave modulations in the low frequency limit: electric permittivity,
magnetic permeability and magnetoelectric coupling. Remarkably, we show that
the theory is exact at all frequencies in the absence of back-reflections, and
exact at low frequencies when that condition is relaxed. This allows us to
derive exact formulae for the Fresnel drag experienced by light travelling
through travelling-wave modulations of electromagnetic media.
