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We study the {\em propagation of electromagnetic waves} in a spacetime devoidof a metric but equipped with a {\em linear} electromagnetic spacetime relation$H\sim\chi\cdot F$. Here $H$ is the electromagnetic excitation $({\cal D},{\calH})$ and $F$ the field strength $(E,B)$, whereas $\chi$ (36 independentcomponents) characterizes the electromagnetic permittivity/permeability ofspacetime. We derive analytically the corresponding Fresnel equation and showthat it is always quartic in the wave covectors. We study the `Fresnel tensordensity' ${\cal G}^{ijkl}$ as (cubic) function of $\chi$ and identify theleading part of $\chi$ (20 components) as indispensable for light propagation.Upon requiring electric/magnetic reciprocity of the spacetime relation, theleading part of $\chi$ induces the {\em light cone} structure of spacetime (9components), i.e., the spacetime metric up to a function. The possibleexistence of an Abelian {\em axion} field (1 component of $\chi$) and/or of a{\em skewon} field (15 components) and their effect on light propagation isdiscussed in some detail. The newly introduced skewon field is expected to beT-odd and related to dissipation.Comment: 16 pages, in Latex, style World Scientific ws-ijmpd.cls, title  changed, one new section added on "axion and skewon", text updated, will  appear in Int.J.Mod.Phys.

GENERALLY COVARIANT FRESNEL EQUATION AND THE EMERGENCE OF THE LIGHT CONE STRUCTURE IN LINEAR PRE-METRIC ELECTRODYNAMICS